Well, I can see a possibility for the 3 letter synonym for 'pch' (starting with the vowel) but Chambers has a slightly different phonetic symbol for its first letter to that of the penultimate syllable of the answer (phonetcally, the answer doesn't sound the 't', so the final syllable works fine).
However, this doesn't seem to improve the surface reading very much, if at all. In fact I'm slightly happier with the synonymous ''?n? p??ch'' (for ''a ...'') than I am with the phonetic ''?n ?p?ch'', which would suggest ''an ...'' for the last two syllables.
That said, many of the 'clues' here are meticulous, so the doubts about this one suggests there's a better idea altogether, so a further hint from you would be very helpful (if you have another idea).
It's particularly tortuous discussing this puzzle - I think this will be my last contribution (apart from 'thank you'!).