A fellow-expert? The cheek! (4)
I believe the answer is:
face
'the cheek' is the definition.
('face' can be a synonym of 'cheek')
'a fellow-expert?' is the wordplay.
'a fellow' becomes 'F' (abbreviation - of a society etc.).
'expert?' becomes 'ace' (ace can informally mean very good).
'f'+'ace'='FACE'
(Other definitions for face that I've seen before include "TV show" , "well-known person" , "Front -- confront" , "steep side of mountain" , "celebrity" .)