Fellow with troubles breaks down (5)
I believe the answer is:
fails
'breaks down' is the definition.
('fail' can be a synonym of 'break down')
'fellow with troubles' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'with' is a charade indicator (letters next to each other).
'troubles' becomes 'ails' (as verb).
'f'+'ails'='FAILS'
(Other definitions for fails that I've seen before include "Does not succeed" , "Doesn't pass" , "Is unsuccessful" .)