Fail to justify fellow's conviction (6)
I believe the answer is:
belief
'conviction' is the definition.
(thesaurus)
'fail to justify fellow's' is the wordplay.
'fail to justify' becomes 'belie' (I've seen this before).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'belie'+'f'='BELIEF'
(Other definitions for belief that I've seen before include "Faith, doctrine" , "Tenet, creed" , "Trust, faith (6)" , "Persuasion" , "Conviction, faith" .)