Crossword Genius

Grouse is good mature (5)

Ross

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" .)

I've seen this clue in The Telegraph.
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