Cheerful star, nominally on vacation (5)
I believe the answer is:
sunny
'cheerful' is the definition.
('sunny' can be similar in meaning to 'cheerful')
'star nominally on vacation' is the wordplay.
'star' becomes 'sun' (Sun is an example).
'on vacation' means to remove the middle letters.
'nominally' with its centre taken out is 'ny'.
'sun'+'ny'='SUNNY'
(Other definitions for sunny that I've seen before include "Bright and cheerful" , "cheerful sound" , "Genial, cheerful" , "Cheerful - cloudless" , "Bright and warm" .)