Union foe (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">union foe</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for scab that I've seen before include "that blocks cut" , "he refuses to come out" , "He's not out" , "Dried crust on wound" , "cover" .)