A criminal way to treat pipes? (3)

I believe the answer is:

lag

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'a criminal' is the definition.
(I've seen this in other clues)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for lag that I've seen before include "trail" , "Insulate (pipes)" , "Fall behind with gal" , "Convict (slang)" , "Wrap to protect from frost" .)

I've seen this clue in the Evening Standard.

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