A relic could be this sweet pastry (6)
I believe the answer is:
eclair
'sweet pastry' is the definition.
'eclair' can be an answer for 'pastry' (eclair is a kind of pastry). I'm not certain of the 'sweet' bit.
'a relic could be' is the wordplay.
'could be' indicates anagramming the letters.
'a'+'relic'='arelic'
'arelic' anagrammed gives 'ECLAIR'.
'this' acts as a link.
Can you help me to learn more?
(Other definitions for eclair that I've seen before include "Choux pastry cream cake" , "Long thin cake with chocolate and cream" , "Cream bun" , "A long thin cake with chocolate covering" , "Type of cream cake" .)