Fine character is reasonable (4)
I believe the answer is:
fair
'is reasonable' is the definition.
(associated in meaning)
'fine character' is the wordplay.
'fine' becomes 'f' (abbreviation).
'character' becomes 'air' (air can mean a character or mood).
'f'+'air'='FAIR'
(Other definitions for fair that I've seen before include "Equitable - market - not bad" , "Blonde; beautiful" , "All clues should be" , "Impartial, so-so" , "Women are known as the ... sex" .)