Crust over a wound (4)
I believe the answer is:
scab
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">crust over a wound</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for scab that I've seen before include "Evidence of healing wound - crosser of picket line" , "Disease of sheep" , "work when others are out" , "evidence of recent injury" , "Blackleg, strike-breaker" .)