Almost backs up strike breaker (4)

I believe the answer is:

scab

I'm a little stuck... Click here to teach me more about this clue!

'strike breaker' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for scab that I've seen before include "work when others are out" , "Crust forming over wound" , "Dried surface of wound" , "Derogatory term for a strike-breaker" , "part of healing process" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.