Clandestine move for bad crook (4,4)

I believe the answer is:

back door

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

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

'move for bad crook' is the wordplay.
'bad crook' anagrams to 'BACK DOOR'.
But, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for back door that I've seen before include "Clandestine" , "Secret or underhand - through rear entrance" , "Underhand means of entry?" .)

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