Prime cheese fellow is after (5)
I believe the answer is:
brief
'prime' is the definition.
(thesaurus)
'cheese fellow is after' is the wordplay.
'cheese' becomes 'brie' (Brie is an example).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'is after' means one lot of letters go next to another.
'brie'+'f'='BRIEF'
(Other definitions for brief that I've seen before include "Terse" , "description of Macbeth's candle" , "Concise; short-lived" , "Remit" , "Case given to a barrister" .)