Oscar's best mate? (2,5,7)
I believe the answer is:
an ideal husband
'best mate?' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for an ideal husband that I've seen before include "the perfect match?" , "best mate?" , "Dramatic scenes involving perfect groom" , "Play by Oscar Wilde" , "best man?" .)