Healing crust (4)
I believe the answer is:
scab
'healing crust' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scab that I've seen before include "sign of wound healing" , "Dried surface of wound" , "one crossing picket line" , "Sore - blackleg" , "Derogatory term for a strike-breaker" .)