Empress having a strain treated (7)
I believe the answer is:
tsarina
'empress' is the definition.
(I know that tsarina is a type of empress)
'a strain treated' is the wordplay.
'treated' is an anagram indicator.
'a'+'strain'='astrain'
'astrain' with letters rearranged gives 'TSARINA'.
'having' acts as a link.
(Other definitions for tsarina that I've seen before include "Catherine, say" , "top Russian women" , "Consort" , "titled lady" , "Wife of an old Russian emperor" .)