State is mean when lacking maturity (4)
I believe the answer is:
aver
'state' is the definition.
(both can mean to declare)
'mean when lacking maturity' is the wordplay.
'mean' becomes 'average' (synonyms).
'when lacking' is a deletion indicator.
'maturity' becomes 'age' (I've seen this in another clue).
'average' with 'age' removed is 'AVER'.
'is' acts as a link.
(Other definitions for aver that I've seen before include "State firmly - don't rave" , "Formally state" , "Vera could declare to be true" , "'Affirm, assert (4)'" , "Maintain - state" .)