Souvenir of rare lichen (5)
I believe the answer is:
relic
'souvenir' is the definition.
(I know that souvenir can be written as relic)
'of rare lichen' is the wordplay.
'of' says the answer is hidden in the clue.
'RELIC' can be found hidden inside 'rare lichen'.
(Other definitions for relic that I've seen before include "Saint's body part?" , "Item surviving from the past" , "Venerated memento" , "Corporal remains" , "Something surviving but outdated" .)