Small part for a big star (5)
I believe the answer is:
cameo
'small part for a big star' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cameo that I've seen before include "Small part for star actor" , "Small role for a well-known actor" , "relief work" , "Small acting role; item of jewellery" , "Piece of jewellery; small role" .)