The stuff of pure pleasure (3)
I believe the answer is:
rep
'the stuff' is the definition.
(I've seen this before)
'of pure pleasure' is the wordplay.
'of' indicates a hidden word.
'REP' can be found hidden inside 'pure pleasure'.
(Other definitions for rep that I've seen before include "Corded cloth for short theatrical group" , "Salesperson" , "agent" , "Fabric with a ribbed surface" , "Repertory theatre" .)