Grouse is good mature (5)
I believe the answer is:
gripe
'grouse' is the definition.
('gripe' can be a synonym of 'grouse')
'good mature' is the wordplay.
'good' becomes 'g' (abbreviation).
'mature' becomes 'ripe' ('ripe' can be similar in meaning to 'mature').
'g'+'ripe'='GRIPE'
'is' acts as a link.
(Other definitions for gripe that I've seen before include "Protest" , "Grumble (colloq.)" , "Complain; colic (water)" , "Trivial complaint" , "Grumble informally" .)