Agent with song to fix (6)
I believe the answer is:
repair
'fix' is the definition.
('repair' can be a synonym of 'fix')
'agent with song' is the wordplay.
'agent' becomes 'rep' (short for representative).
'with' is a charade indicator (letters next to each other).
'song' becomes 'air' (I've seen this before**).
'rep'+'air'='REPAIR'
'to' acts as a link.
(Other definitions for repair that I've seen before include "Patch up" , "retire" , "Restore (something) to good condition" , "Restore to working condition" , "Mend, restore to good condition" .)