Crossword Genius

Fancy pants in use, old (8)

Ross

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" .)

I've seen this clue in The Telegraph.
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