Incarcerating the lawbreaker, beginning to be firm (8)

I believe the answer is:

stalwart

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'to be firm' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

(Other definitions for stalwart that I've seen before include "Resolute and firm" , "Valiant" , "Walt's rat is strong and brave" , "Sturdy, tenacious and tough" , "Determined" .)

I've seen this clue in the Evening Standard.

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