Ostentatious fellow with whip (5)
I believe the answer is:
flash
'ostentatious' is the definition.
(I've seen this before)
'fellow with whip' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'with' is a charade indicator (letters next to each other).
'whip' becomes 'lash' (synonyms).
'f'+'lash'='FLASH'
(Other definitions for flash that I've seen before include "Mo" , "Ostentatious" , "emblem on uniform" , "Tasteless" , "Ostentatiously stylish" .)