Pot mostly in the air: I refuse to fill it (2-3)
I believe the answer is:
in-off
'pot' is the definition.
(an in-off is a way of potting a ball in billiards)
'mostly in the air i refuse to fill it' is the wordplay.
'mostly' means to remove the last letter.
'in the air' becomes 'iffy' (both can mean uncertain or undecided).
'i refuse' becomes 'no'.
'to fill it' means one lot of letters goes inside another (back).
'iffy' with its final letter removed is 'iff'.
'iff' enclosing 'no' is 'IN-OFF'.
(Other definitions for in-off that I've seen before include "Glancing billiards pot" , "Snooker ball sunk having hit another" , "Snooker hazard" , "Foul (snooker), scoring stroke (billiards)" , "Shot that's good at billiards and foul at snooker" .)