Grouse without starter arrived at perfection (4)
I believe the answer is:
ripe
'arrived at perfection' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives occasionally define each other.
'grouse without starter' is the wordplay.
'grouse' becomes 'gripe' (synonyms).
'without starter' means to remove the first letter.
'gripe' with its first letter taken away is 'RIPE'.
Can you help me to learn more?
(Other definitions for ripe that I've seen before include "Mellow" , "Fully developed or matured" , "Opportune" , "Mature, ready to eat" , "Mature, fully grown" .)