Relic I discovered in a rooftop crater (8)
I believe the answer is:
archaism
'relic' is the definition.
(thesaurus)
'i discovered in a rooftop crater' is the wordplay.
'discovered in' means one lot of letters goes inside another.
'rooftop' becomes 'r' (top of the word 'roof').
'crater' becomes 'chasm' (I can't explain this - if you can you should believe this answer much more).
'a'+'r'+'chasm'='archasm'
'i' placed inside 'archasm' is 'ARCHAISM'.
Can you help me to learn more?
(Other definitions for archaism that I've seen before include "Out-of-date word" , "Ancient word or expression for charisma" , "Something in an out of date style" , "Behold, perhaps" , "Charisma (anag) - something very old-fashioned" .)