Why the hard-bitten crook gave himself up? (6,3)

I believe the answer is:

police dog

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

'why' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for police dog that I've seen before include "busy helper" , "maybe I do a lot of sniffing" , "Canine for the boys in blue" , "Officer's co-worker" , "Force assistant" .)

I've seen this clue in the Evening Standard.

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.