Not on strike? That's unusual! (7)
I believe the answer is:
offbeat
'unusual' is the definition.
(thesaurus)
'not on strike?' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'strike?' becomes 'beat' (I've seen this before).
'off'+'beat'='OFFBEAT'
'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" .)