Acted the part of Oscar in small role (5)
I believe the answer is:
cameo
'small role' is the definition.
(I've seen this in another clue)
'acted the part of oscar' is the wordplay.
'acted' becomes 'came' (I can't explain this - if you can you should give a lot more credence to this answer).
'the part of' indicates taking the first letters (I've seen 'part' mean this).
The first letter of 'oscar' is 'o'.
'came'+'o'='CAMEO'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for cameo that I've seen before include "Item of jewellery with a portrait - distinguished actor's minor role" , "Short performance" , "Brooch with an engraving in low relief" , "Jewellery item - small dramatic role" , "Famous actor's small role" .)