Small part arrived, leading to Oscar (5)
I believe the answer is:
cameo
'small part' is the definition.
(small part in a film)
'arrived leading to oscar' is the wordplay.
'arrived' becomes 'came' (come can mean to appear or arrive).
'leading to' is a charade indicator (letters next to each other) (some letters go before or lead others).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'came'+'o'='CAMEO'
(Other definitions for cameo that I've seen before include "Miniature picture" , "Part in movie" , "Brooch with an engraving in low relief" , "Small part for star actor" , "Medallion with a profile carved in relief" .)