Wound crust (4)
I believe the answer is:
scab
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wound crust</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for scab that I've seen before include "Dried crust on wound" , "Dried crust on sore" , "Crusted sore on cut or wound" , "Derogatory term for a strike-breaker" , "He's not out" .)