Wild lion used in fantasy (8)
I believe the answer is:
delusion
'fantasy' is the definition.
(thesaurus)
'wild lion used' is the wordplay.
'wild' indicates an anagram.
'lion'+'used'='lionused'
'lionused' anagrammed gives 'DELUSION'.
'in' acts as a link.
(Other definitions for delusion that I've seen before include "Misguided belief" , "Misapprehension" , "Unsoiled kind of hallucination" , "Belief held in the face of all contrary evidence" , "Unsoiled sort of hallucination" .)