Pretty loud tune (4)
I believe the answer is:
fair
'pretty' 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'
(Other definitions for fair that I've seen before include "Just treatment of others" , "Traders' gathering" , "Pretty good" , "All clues should be" , "Just; sunny" .)