Loewe's partner (6)
I believe the answer is:
lerner
'loewe's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lerner that I've seen before include "US lyricist" , "American lyricist" , "4 1 [THANK HEAVENS FOR LITTLE GIRLS] writer" , "Musical librettist" .)