Souvenir of pure lichen (5)
I believe the answer is:
relic
'souvenir' is the definition.
(I know that souvenir can be written as relic)
'of pure lichen' is the wordplay.
'of' says the answer is hidden in the clue.
'RELIC' is hidden within 'pure lichen'.
(Other definitions for relic that I've seen before include "Left over item, sometimes sacred" , "Surviving artefact" , "Memorial of antiquity" , "Hangover from an earlier time" , "Ossified survival" .)