Crossword Genius

Criminals one may be as thick as (7)

Ross

I believe the answer is:

thieves

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

'criminals' is the definition.
(thief is a kind of criminal)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for thieves that I've seen before include "People who steal" , "they whip things" , "Crooks" , "Stealers" , "They pinch" .)

I've seen this clue in The Mirror.
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