Bill's partner in love (3)
I believe the answer is:
coo
'bill's partner in love' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for coo that I've seen before include "I'm surprised" , "Pigeon's sound" , "(Of pigeon) murmur" , "Cry of doves - cry of surprise?" , "Gosh!" .)