Good mature beef (5)
I believe the answer is:
gripe
'beef' is the definition.
(I know that beef is a type of beef)
'good mature' is the wordplay.
'good' becomes 'g' (abbreviation).
'mature' becomes 'ripe' (synonyms).
'g'+'ripe'='GRIPE'
(Other definitions for gripe that I've seen before include "Complain; stomach pain" , "Protest" , "Grumble (colloq.)" , "Complain; colic (water)" , "Complain, grouse" .)