Sir's opposite (5)
I believe the answer is:
madam
'sir's opposite' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for madam that I've seen before include "pompous young lady?" , "Courteous form of address to a woman" , "Palindrome used in speaking politely to a woman" , "Term of address to a woman" , "Polite form of address for a woman" .)