Slyly spiteful (5)
I believe the answer is:
catty
'slyly spiteful' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for catty that I've seen before include "Bitchy as a feline" , "Spiteful in feline way" , "Backbiting" , "Mean and spiteful" , "It's like a queen" .)