Mature, heading off complaint (4)
I believe the answer is:
ripe
'mature' is the definition.
(I know that mature can be written as ripe)
'heading off complaint' is the wordplay.
'heading off' means to remove the first letter.
'complaint' becomes 'gripe' (an objection or source of annoyance).
'gripe' with its initial letter taken off is 'RIPE'.
(Other definitions for ripe that I've seen before include "Mature, ready for harvesting" , "green? No!" , "At fruition" , "Ready (to be eaten or used)" , "Seasoned, fully grown" .)