Initiate action on a plane, perhaps? (6,2)
I believe the answer is:
strike up
'initiate' is the definition.
(as in striking up or initiating a friendship)
'action on a plane perhaps?' is the wordplay.
'action' becomes 'strike' (strike is a kind of action).
'on a plane perhaps?' becomes 'up' (up in the air).
'strike'+'up'='STRIKE-UP'
(Other definitions for strike up that I've seen before include "Begin to play (a piece of music)" , "Start playing music - begin (acquaintance) - emboss" .)