Picket line crosser (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">picket line crosser</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)