Memento of derelict mansion (5)
I believe the answer is:
relic
'memento' is the definition.
(a relic is an antique or memento)
'of derelict mansion' is the wordplay.
'of' says the answer is hidden in the clue.
'RELIC' can be found hidden inside 'derelict mansion'.
(Other definitions for relic that I've seen before include "Venerated memento" , "church treasure" , "Survival from earlier times" , "Something from saint" , "Survival from an earlier time" .)