Must trim large, fashionable cloth (6)
I believe the answer is:
muslin
'cloth' is the definition.
(muslin is a type of cloth)
'must trim large fashionable' is the wordplay.
'trim' means to remove the last letter (I've seen 'trimmed' mean this (the final letter is trimmed off)).
'large' becomes 'L' (eg in clothes sizes).
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'must' with its final letter taken away is 'mus'.
'mus'+'l'+'in'='MUSLIN'
(Other definitions for muslin that I've seen before include "Cotton for veil or strainer" , "dress material" , "Lightweight cotton fabric" , "Light cotton cloth" , "Gauze-like cloth" .)