Bill's companion? Goodness me! (3)
I believe the answer is:
coo
I believe this clue is a double definition.
'bill's companion?' is the first definition.
(I've seen this before)
'goodness me' is the second definition.
(exclamation of surprise)
(Other definitions for coo that I've seen before include "Speak lovingly -- cry softly" , "Murmur amorously" , "Soft murmuring sound of a pigeon" , "Soft sound of dove or baby" , "Breathe" .)