Rogues serious about theory (5)

I believe the answer is:

guess

'theory' is the definition.
(thesaurus)

'rogues serious about' is the wordplay.
'about' says the answer is hidden in the clue.
'GUESS' is hidden in the letters of 'rogues serious'.

(Other definitions for guess that I've seen before include "Estimate, supposition" , "Fancy" , "Estimate; suppose" , "Reckon" , "Imagine" .)

I've seen this clue in The Mirror.

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