Fawcett (2010: 205-6, 206n):
Indeed, we might well ask what evidence there is to support the claim that a phrase is "a contraction of a clause". Halliday gives none, so we must look for it ourselves. (As we do so, however, we shall also dig up evidence for taking the contrary view.)
The strongest evidence that I can think of is the fact that, when a prepositional group is functioning as the qualifier in nominal group, it often seems possible to 'expand' it to a clause. For example, the last two words of the title of the popular British TV programme Neighbours from Hell are a prepositional group or phrase, and this can be "expanded" into a clause by adding who come so that it reads Neighbours who come from Hell.
However, if we follow this line of argument to its logical conclusion, we shall find ourselves deriving all modifiers from a clause that fills the qualifier, e.g., friendly neighbours from neighbours who are friendly — and so on for many other types of modifier. In other words, if Halliday is going to relate prepositional groups functioning as qualifiers to clauses, why not do the same with quality groups that are functioning as modifiers? …
Moreover, even if we consider only prepositional groups functioning as qualifiers, there are examples such as young men with long hair that cannot be 'expanded' (i.e., we shall not find examples such as young men who are with long hair).
Finally, consider the even more frequent use of prepositional groups when they function as clause elements. In such cases there is normally no possibility of 'expanding' the group or phrase to a clause. For example, we can say I'll put it in the box, but we cannot expand in the box in any natural way into a clause.¹⁰
¹⁰ It is true that we can replace a prepositional group such as at five o'clock by a clause in I'll call round when it's five o'clock — but that is a replacement rather than an expansion. The utterance I'll call round when it's at five o'clock sounds very odd, and I'll put it where it's in the box sounds if anything even odder.
The conclusion must be that, within the linguistic description of a text, we should treat prepositional groups as syntactic units in their own right. In other words, we should not try to capture at the level of syntax the fact that the referents of young men with long hair are (probably) the same as the referents of young men who have long hair.
A prepositional phrase can be interpreted as a shrunken clause, in which the preposition serves as a ‘minor process’, interpreted as a kind of mini-verb, and the nominal group as a participant in this minor process. This needs explaining.The preposition, it was suggested, acts as a kind of intermediary whereby a nominal element can be introduced as an ‘indirect’ participant in the main process. We saw also that in circumstantial and possessive relational processes there are often close parallels between be + preposition and a verb, e.g.the delay was because of a strike ~ was caused by a strikea carpet was over the floor ~ covered the floorthe bridge is across the river ~ crosses/spans the rivera path is along(side) the wood ~ skirts the wooda halo is around the moon ~ surrounds the moonThis similarity between verb and preposition can also be seen in cases where there is a close relationship between a prepositional phrase and a non-finite dependent clause:he cleaned the floor with a mop ~ using a mopgrass grows after the rain ~ following the rainIn this way certain prepositions are themselves derived from non-finite verbs; e.g. concerning, according to, given, excepting. These considerations suggest that the nominal group stands to the preposition in some kind of transitivity relation, as well as in a relationship like that of Complement to Predicator in mood structure.