One who crosses the line? (4)
I believe the answer is:
scab
'one who crosses the line?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scab that I've seen before include "Blackleg - sign of injury" , "cover" , "Crust formed over wound" , "Crust forming over wound" , "Strike breaker" .)