A fellow joining sizeable event (6)
I believe the answer is:
'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).
Can you help me to learn more?
(Other definitions for affair that I've seen before include "Romantic relationship between two unmarried people", "Business; romance", "Secret sexual relationship", "A function or a romance", "Liaison or intrigue".)