Fellow, a quiet one, on trend (7)
I believe the answer is:
fashion
'trend' is the definition.
(I know that fashion is a type of trend)
'fellow a quiet one on' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'one' becomes 'I' (Roman numeral).
'f'+'a'+'sh'+'i'+'on'='FASHION'
(Other definitions for fashion that I've seen before include "way" , "Haute couture" , "On a fish (anag.)" , "Vogue, trend" , "Current popular style" .)