Bill's partner in dalliance (3)

I believe the answer is:

coo

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'bill's partner' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for coo that I've seen before include "Murmur amorously" , "Dove's call" , "Dove's noise" , "Pigeon call" , "Cry of doves - cry of surprise?" .)

I've seen this clue in The Globe and Mail.

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