Fairy's cattiness about Romeo (6)
I believe the answer is:
sprite
'fairy's' is the definition.
('sprite' can be a synonym of 'fairy')
'cattiness about romeo' is the wordplay.
'cattiness' becomes 'spite' ('spite' can be a synonym of 'cattiness').
'about' means one lot of letters goes inside another.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'spite' going around 'r' is 'SPRITE'.
(Other definitions for sprite that I've seen before include "Puck, say" , "Nimble elf-like creature - icon in computer game" , "Elf, fairy" , "Small, playful fairy" , "Brownie" .)