'You are really naughty' starts the story (4)
I believe the answer is:
yarn
'the story' is the definition.
('yarn' can be a synonym of 'story')
'you are really naughty starts' is the wordplay.
'starts' suggests taking the first letters.
The initial letters of 'you are really naughty' is 'YARN'.
(Other definitions for yarn that I've seen before include "Sham" , "Rope" , "that'll keep you in stitches?" , "Cord of twisted fibres, wool say" , "Story, and thread or wool, say" .)