Cheerful star, extremely naughty (5)
I believe the answer is:
sunny
'cheerful' is the definition.
(similar in meaning)
'star extremely naughty' is the wordplay.
'star' becomes 'sun' (the Sun is a star).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'naughty' with its middle taken out is 'ny'.
'sun'+'ny'='SUNNY'
(Other definitions for sunny that I've seen before include "Cheerful; fine (weather)" , "Bright and warm" , "Cheerful - cloudless" , "Unclouded - cheerful" , "cheerful sound" .)