Bill's partner left with it (4)
I believe the answer is:
cool
'with it' is the definition.
(I've seen this in another clue)
'bill's partner left' is the wordplay.
'bill's partner' becomes 'coo' (I've seen this in another clue).
'left' becomes 'l' (common abbreviation).
'coo'+'l'='COOL'
(Other definitions for cool that I've seen before include "Unfriendly; excellent" , "Unemotional; chill" , "Sangfroid" , "Calm; chilly" , "Calm and unemotional" .)