Fancy pants in use, old (8)
I believe the answer is:
delusion
'fancy' is the definition.
(thesaurus)
'pants in use old' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'in'+'use'+'old'='inuseold'
'inuseold' is an anagram of 'DELUSION'.
(Other definitions for delusion that I've seen before include "Misguided belief" , "Misapprehension" , "False or mistaken belief" , "Mistaken belief in the face of contradictory evidence" , "Unsoiled sort of hallucination" .)