Talk spitefully about tail and sting (8)
I believe the answer is:
backbite
'talk spitefully about' is the definition.
(to backbite is to talk about someone maliciously)
'tail and sting' is the wordplay.
'tail' becomes 'back'.
'and' says to put letters next to each other.
'sting' becomes 'bite' (synonyms).
'back'+'bite'='BACKBITE'
(Other definitions for backbite that I've seen before include "Speak unkindly of (someone) in their absence" , "Say spiteful things about an absentee" , "Talk spitefully about absent person" , "Gossip maliciously" , "Talk maliciously - from behind?" .)