Start playing when industrial action finished (6,2)
I believe the answer is:
strike up
'start playing' is the definition.
(synonyms)
'industrial action finished' is the wordplay.
'industrial action' becomes 'strike' (workers going on strike).
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'strike'+'up'='STRIKE-UP'
'when' is the link.
(Other definitions for strike up that I've seen before include "Start playing music - begin (acquaintance) - emboss" , "Begin to play (a piece of music)" .)