Fold left trapped by decaying vegetable matter (5)
I believe the answer is:
pleat
'fold' is the definition.
(I know that pleat is a type of fold)
'left trapped by decaying vegetable matter' is the wordplay.
'left' becomes 'l' (common abbreviation).
'trapped by' is an insertion indicator.
'decaying vegetable matter' becomes 'peat'.
'l' placed within 'peat' is 'PLEAT'.
(Other definitions for pleat that I've seen before include "that's tailor-made" , "Fold in material" , "Crease" , "Flat fold of cloth" , "Tuck" .)