Picket line crosser (4)
I believe the answer is:
'picket line crosser' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scab that I've seen before include "Dried crust on sore" , "cause of itch?" , "Non-striker" , "Blackleg, strike-breaker" , "Dried surface of wound" .)