Supernatural creature (6)
'supernatural creature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for elf that I've seen before include "Brownie" , "Goblin" , "Folk tale creature" , "spirit" , "Imaginary small being of selfish kind" .)
(Other definitions for goblin that I've seen before include "Malevolent gnome" , "Little monster" , "Small trouble-making creature of folklore" , "I may appear in fairy tales" , "Naughty sprite" .)