Handle subject of diplomatic negotiations but not completely (5)
I believe the answer is:
treat
'handle' is the definition.
(I know that handle can be written as treat)
'subject of diplomatic negotiations but not completely' is the wordplay.
'subject of diplomatic negotiations' becomes 'treaty' (a treaty might be negotiated over).
'but not completely' means to remove the last letter.
'treaty' with its final letter taken away is 'TREAT'.
(Other definitions for treat that I've seen before include "Celebration" , "Pay for someone's food or entertainment" , "Give care to; surprise gift" , "Deal with; an outing" , "Practise medicine" .)