Crust over a wound (4)
I believe the answer is:
scab
'crust over a wound' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)