Walk-on role (5)
I believe the answer is:
cameo
'walk-on role' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cameo that I've seen before include "Carving in low relief" , "Short performance" , "Piece of jewellery with carved portrait" , "E. Med. nation" , "Small role for a well-known actor" .)