Reality show supporting fellow (4)
I believe the answer is:
fact
'reality' is the definition.
(fact is a kind of reality)
'show supporting fellow' is the wordplay.
'show' becomes 'act' (act is a kind of show**).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'act' put after 'f' is 'FACT'.
(Other definitions for fact that I've seen before include "Piece of information known to be true" , "Undisputed item" , "Indisputable thing" , "Something indisputable" , "Actuality" .)