Sash that's too big trimmed to fit (3)
I believe the answer is:
obi
'sash' is the definition.
(type of Japanese sash or belt)
'that's too big trimmed to fit' is the wordplay.
'trimmed to fit' indicates the answer is hidden within the clue (the words are trimmed down).
'OBI' is hidden within 'thats too big'.
(Other definitions for obi that I've seen before include "belt" , "Kimono accessory" , "sash from Tokyo" , "Kimono sash; W. African sorcery" , "Material band" .)