Recess in rap session (4)
I believe the answer is:
apse
'recess' is the definition.
(apse is a kind of recess)
'in rap session' is the wordplay.
'in' indicates the answer is hidden within the clue.
'APSE' is hidden in the letters of 'rap session'.
(Other definitions for apse that I've seen before include "Recess in church wall" , "part of cathedral" , "In sacred space" , "Peas (anag) - found in church" , "space for altar" .)