Leader of orchestra will acknowledge applause (4,1,3)
I believe the answer is:
take a bow
I believe this clue is a double definition.
'leader of orchestra will' is the first definition.
(the first violinist uses a bow)
'acknowledge applause' is the second definition.
(synonyms)
(Other definitions for take a bow that I've seen before include "Acknowledge praise or accept credit" , "What Menuhin would initially do" , "Violinist's lesson one" , "Acknowledge applause as entertainer does" , "Acknowledge applause with physical posture" .)