Waste material rotten, bitter mostly (5)
I believe the answer is:
offal
'waste material' is the definition.
(I've seen this before)
'rotten bitter mostly' is the wordplay.
'rotten' becomes 'off' (I've seen this before).
'bitter' becomes 'ale' ('ale' can be a synonym of 'bitter').
'mostly' means to remove the last letter.
'ale' with its final letter taken off is 'al'.
'off'+'al'='OFFAL'
(Other definitions for offal that I've seen before include "Waste parts of a carcass" , "Putting cargo on (ship)" , "Entrails used as food" , "Edible (or inedible?) internal organs" , "Refuse - edible entrails of animal" .)