Loud tune is not bad (4)
I believe the answer is:
fair
'not bad' is the definition.
(I've seen this before)
'loud tune' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'tune' becomes 'air' (synonyms).
'f'+'air'='FAIR'
'is' acts as a link.
(Other definitions for fair that I've seen before include "Traders' gathering" , "Just; beautiful; blond" , "Average" , "[OPPOSITE OF] Dark" , "Equitable or blond" .)