Fail to justify fellow's trust (6)
I believe the answer is:
belief
'trust' is the definition.
(I've seen this before)
'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; confidence" , "Trust, faith (6)" , "Tenet, creed" , "Religious conviction" , "Credo" .)