About to enjoy oneself when it turns bright (6)
I believe the answer is:
clever
'bright' is the definition.
(bright can mean clever or intelligent)
'about to enjoy oneself when it turns' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to enjoy oneself' becomes 'revel' (to revel is to party or celebrate).
'when it turns' is a reversal indicator.
'revel' back-to-front is 'lever'.
'c'+'lever'='CLEVER'
(Other definitions for clever that I've seen before include "Gifted" , "Quick and intelligent" , "Box ---, outwit someone" , "Quick, skilful" , "expert" .)