Souvenir of pure licentiousness (5)
I believe the answer is:
relic
'souvenir' is the definition.
(synonyms)
'of pure licentiousness' is the wordplay.
'of' indicates the answer is hidden within the clue.
'RELIC' can be found hidden inside 'pure licentiousness'.
(Other definitions for relic that I've seen before include "Keepsake, of a saint say" , "One interestingly old" , "Piece left over" , "Surviving ancient custom" , "One may venerate this" .)