Not on strike? That's unusual! (7)
I believe the answer is:
'unusual' is the definition.
'not on strike?' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'strike?' becomes 'beat' (I've seen this before).
'that's' is the link.
(Other definitions for offbeat that I've seen before include "Weird - irregular" , "Unusual, not ordinary - or rhythmic" , "rather unorthodox" , "Way-out" , "Wacky, eccentric" .)