The devil's bunch regularly occupying prisoner (7)
I believe the answer is:
lucifer
'the devil's' is the definition.
('Lucifer' can be a synonym of 'devil')
'bunch regularly occupying prisoner' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'occupying' is an insertion indicator.
'prisoner' becomes 'lifer' (lifer is a kind of prisoner).
The alternate letters of 'bunch' are 'uc'.
'uc' going within 'lifer' is 'LUCIFER'.
(Other definitions for lucifer that I've seen before include "Light" , "striker" , "Evil spirit" , "Rising star" , "flares up when struck" .)