Sassy pro bumping off former partner (4)
I believe the answer is:
pert
'sassy' is the definition.
(thesaurus)
'pro bumping off former partner' is the wordplay.
'pro' becomes 'expert' (I've seen this before).
'bumping off' is a deletion indicator.
'former partner' becomes 'ex'.
'expert' with 'ex' taken out is 'PERT'.
(Other definitions for pert that I've seen before include "Impudent, cheeky" , "Saucy, maybe brash" , "Jaunty" , "Irreverent or saucy" , "Saucy and chipper" .)