Roguish as a duke, for instance? (4)

I believe the answer is:

arch

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

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

I can't explain the remainder of the clue.

(Other definitions for arch that I've seen before include "Curve spanning an opening" , "Way though - on foot!" , "spanner" , "Curved part of the foot" , "This enemy is major" .)

I've seen this clue in the Evening Standard.

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