Be in charge and shout to the melody (4,3,4)
I believe the answer is:
call the tune
'be in charge' is the definition.
(I've seen this in another clue)
'shout to the melody' is the wordplay.
'shout' becomes 'call' (synonyms).
'melody' becomes 'tune' (synonyms).
'call'+'the'+'tune'='CALL THE TUNE'
'and' is the link.
(Other definitions for call the tune that I've seen before include "Take charge in making a decision" , "Control the proceedings" , "Be in command" , "order" .)