Wound crust (4)
I believe the answer is:
scab
'wound crust' is the definition.
(I've seen this before)
This is all the clue.
(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" .)