Clothier’s forbidden more trim (9)
I believe the answer is:
outfitter
'clothier's' is the definition.
(I've seen this before)
'forbidden more trim' is the wordplay.
'forbidden' becomes 'out' ('out' can be a synonym of 'forbidden').
'more trim' becomes 'fitter' ('fit' can be a synonym of 'trim').
'out'+'fitter'='OUTFITTER'
(Other definitions for outfitter that I've seen before include "Tailor" , "suiting, from him?" , "who could help in gear selection?" , "Seller of clothes" , "Men's clothes shop" .)