Long arm of the law, perhaps? (4)

I believe the answer is:

limb

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'long arm' is the definition.
'limb' can be an answer for 'arm' (I've seen this in another clue). I'm not certain of the 'long' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for limb that I've seen before include "Arm, leg or wing" , "A supporter" , "Branch - arm - leg" , "as part of plane, say" , "Wing, perhaps" .)

I've seen this clue in the Evening Standard.

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