Brave expert supporting fellow (4)
I believe the answer is:
face
'brave' is the definition.
(I've seen this in other clues)
'expert supporting fellow' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'ace' put after 'f' is 'FACE'.
(Other definitions for face that I've seen before include "'Cheeks, mouth, eyes etc (4)'" , "steep side of mountain" , "Surface; visage" , "celebrity" , "Come up against" .)