Souvenir of pure licentiousness (5)
I believe the answer is:
relic
'souvenir' is the definition.
(I know that souvenir can be written as relic)
'of pure licentiousness' is the wordplay.
'of' indicates a hidden word.
'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" .)