Top actress gained ally in play about drama's opening (7,4)
I believe the answer is:
leading lady
'top actress' is the definition.
(lead actress)
'gained ally in play about drama's opening' is the wordplay.
'in play' indicates an anagram.
'about' means one lot of letters goes inside another.
'opening' says to take the initial letters.
The initial letter of 'dramas' is 'd'.
'gained'+'ally'='gainedally'
'gainedally' with letters rearranged gives 'leadinglay'.
'leadinglay' placed around 'd' is 'LEADING LADY'.
(Other definitions for leading lady that I've seen before include "Heroine - gaily landed (anag)" , "Female top of the bill" , "a star" , "Female playing main part in play" , "Film" .)