Criminals one may be as thick as (7)
I believe the answer is:
thieves
'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" .)