Company boss limits amateur's brief appearance (5)
I believe the answer is:
cameo
'brief appearance' is the definition.
(brief appearance in a film)
'company boss limits amateur's' is the wordplay.
'company boss' becomes 'CEO' (chief executive officer).
'limits' means one lot of letters goes inside another (limit can mean to contain or block off).
'amateur' becomes 'am' (as in am-dram).
'ceo' placed around 'am' is 'CAMEO'.
(Other definitions for cameo that I've seen before include "Jewellery item with carved portrait" , "Something small but well done - say role in play" , "Brief role" , "Piece of jewellery; small role" , "Carving in relief, on a brooch say" .)