Knock into the air and begin to play (6,2)
I believe the answer is:
strike up
'begin to play' is the definition.
(as in beginning a piece of music)
'knock into the air' is the wordplay.
'knock' becomes 'strike'.
'into the air' becomes 'up'.
'strike'+'up'='STRIKE UP'
'and' is the link.
(Other definitions for strike up that I've seen before include "Begin to play (a piece of music)" , "Start playing music - begin (acquaintance) - emboss" .)