Antique concealing rare lichen (5)
I believe the answer is:
relic
'antique' is the definition.
(I've seen this before)
'concealing rare lichen' is the wordplay.
'concealing' indicates a hidden word.
'RELIC' can be found hidden inside 'rare lichen'.
(Other definitions for relic that I've seen before include "Outdated survival" , "Venerated memento" , "Left over item, sometimes sacred" , "Holy fragment as of saint" , "for remembrance" .)