A fellow joining sizeable event (6)
I believe the answer is:
affair
'event' is the definition.
(affair is a kind of event)
'a fellow joining sizeable' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'joining' is a charade indicator (letters next to each other).
'sizeable' becomes 'fair' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'f'+'fair'='AFFAIR'
Can you help me to learn more?
(Other definitions for affair that I've seen before include "Romantic relationship, perhaps illicit" , "Thing to be done with raffia" , "Matter or concern, perhaps of the heart" , "Concern, matter or illicit romance" , "See 23" .)