Sit with reformed impostor, having forsaken devil (5)
I believe the answer is:
roost
'sit' is the definition.
(roost can mean to sit (as a bird))
'reformed impostor having forsaken devil' is the wordplay.
'reformed' indicates anagramming the letters.
'having forsaken' is a deletion indicator.
'devil' becomes 'imp' (devilish creature).
'impostor' with 'imp' taken out is 'ostor'.
'ostor' is an anagram of 'ROOST'.
'with' acts as a link.
(Other definitions for roost that I've seen before include "Place on which bird rests" , "The cock rules it" , "Bird's settling place" , "Place where birds perch or settle for sleep" , "Henhouse" .)