Antique concealing rare lichen (5)
I believe the answer is:
relic
'antique' is the definition.
(thesaurus)
'concealing rare lichen' is the wordplay.
'concealing' indicates the answer is hidden within the clue.
'RELIC' is hidden in the letters of '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(Other definitions for relic that I've seen before include "Outdated survival" , "Venerated memento" , "Left over item, sometimes sacred" , "Holy fragment as of saint" , "for remembrance" .)