Held (in) (6)
'held in' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pent that I've seen before include "enclosed" , "Closely confined" .)
(Other definitions for reined that I've seen before include "Pulled in, checked progress" , "Restrained" , "Stopped or slowed a horse" .)