Talk spitefully about side with incisiveness (8)
I believe the answer is:
backbite
'talk spitefully about' is the definition.
(to backbite is to talk about someone maliciously)
'side with incisiveness' is the wordplay.
'side' becomes 'back' (back is a kind of side).
'with' means one lot of letters go next to another.
'incisiveness' becomes 'bite' (I've seen this in another clue).
'back'+'bite'='BACKBITE'
(Other definitions for backbite that I've seen before include "Say mean things" , "Say spiteful things about an absentee" , "Gossip maliciously" , "Speak unkindly of (someone) in their absence" , "Speak ill of (someone) in their absence" .)