Plant found mostly in decaying vegetable matter (3)
I believe the answer is:
pea
'plant' is the definition.
'found mostly in decaying vegetable matter' is the wordplay.
'found mostly in' means to remove the last letter.
'decaying vegetable matter' becomes 'peat'.
'peat' with its last letter taken off is 'PEA'.
(Other definitions for pea that I've seen before include "Spherical vegetable" , "Edible seed from pod" , "Leguminous seed" , "The plant is a strange ape" , "Vegetable in a whistle" .)