Subject of a very loud song (6)
I believe the answer is:
affair
'subject' is the definition.
(I've seen this before)
'a very loud song' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'song' becomes 'air' (I've seen this before).
'a'+'ff'+'air'='AFFAIR'
'of' is the link.
(Other definitions for affair that I've seen before include "Involvement" , "Concern, matter, business" , "Romantic relationship between two unmarried people" , "scandal" , "Raffia (anag)" .)