Fail to justify felon's first conviction (6)
I believe the answer is:
belief
'conviction' is the definition.
(I've seen this before)
'fail to justify felon's first' is the wordplay.
'fail to justify' becomes 'belie' (I've seen this before).
'felon's first' becomes 'f' (1st letter of 'felon').
'belie'+'f'='BELIEF'
(Other definitions for belief that I've seen before include "Faith; confidence" , "Principle accepted without proof" , "Faith, credence" , "Conviction, faith" , "Credo" .)