Fiend and swindler together regularly doing time (6)
I believe the answer is:
inside
'doing time' is the definition.
(I've seen this in another clue)
I don't understand how the remainder of the clue works.
(Other definitions for inside that I've seen before include "Interior - in prison" , "serving porridge" , "locked up" , "A sort of job" , "Middle" .)