Actress mainly crows over Oscar (5)
I believe the answer is:
garbo
'actress' is the definition.
(I know that Garbo is a actor)
'mainly crows over oscar' is the wordplay.
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'crows' becomes 'brags' ('brag' can be a synonym of 'crow').
'over' says the letters should be written backwards.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'brags' with its last letter taken off is 'brag'.
'brag' in reverse letter order is 'garb'.
'garb'+'o'='GARBO'
(Other definitions for garbo that I've seen before include "Greta -, Swed. actress" , "Swedish film star who wanted to be alone" , "Swedish film actress Greta" , "film-star" , "film star? Yes and no" .)