Trendy fellow having made a mistake, it's concluded (8)
I believe the answer is:
inferred
'it's concluded' is the definition.
(inferring is a kind of concluding)
'trendy fellow having made a mistake' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'having' is a charade indicator (letters next to each other).
'made a mistake' becomes 'erred' ('err' can be a synonym of 'make a mistake').
'in'+'f'+'erred'='INFERRED'
(Other definitions for inferred that I've seen before include "Deduced by reasoning" , "'Concluded, judged from the facts (8)'" , "Conjectured" , "Rein Fred in because he deduced" .)