It's a very loud tune, love (6)
I believe the answer is:
affair
'love' is the definition.
(thesaurus)
'it's a very loud tune' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'tune' becomes 'air' (air can mean a piece of music).
'a'+'ff'+'air'='AFFAIR'
(Other definitions for affair that I've seen before include "Illicit liaison" , "A fling (or something more serious?)" , "Vaguely specified concern" , "A matter; amorous episode" , "Former Belgian coin" .)