Begin once more to relax with music, say (7)
I believe the answer is:
restart
'begin once more' is the definition.
('start' can be a synonym for 'begin' so perhaps 'restart' is OK for 'begin once more')
'relax with music say' is the wordplay.
'relax' becomes 'rest' (to relax is to have a rest).
'with' is a charade indicator (letters next to each other).
'music say' becomes 'art' (music is a kind of art).
'rest'+'art'='RESTART'
'to' is the link.
(Other definitions for restart that I've seen before include "launch again" , "Commence again" , "Resume" , "begin afresh" , "Jumping the gun will mean" .)