A renegade sailor's comeuppance (3)

I believe the answer is:

rat

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

'a renegade' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for rat that I've seen before include "Contemptible person" , "Betray like a rodent" , "Unpopular rodent" , "Inform" , "one not going down with ship" .)

I've seen this clue in The Globe and Mail.

Want a hint initially instead of a full solution? Install my app