Fellow with earls changing trousers (6)
I believe the answer is:
flares
'trousers' is the definition.
(I have seen 'Unfashionable trousers' mean 'flares' so perhaps 'trousers' could also mean 'flares')
'fellow with earls changing' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'with' is a charade indicator (letters next to each other).
'changing' indicates anagramming the letters.
'earls' is an anagram of 'lares'.
'f'+'lares'='FLARES'
(Other definitions for flares that I've seen before include "Bell-bottom trousers" , "Wide-bottomed trousers" , "Devices producing bright lights as signal" , "Bursts of bright lights and a trouser style" , "Blazes of light" .)