Cheerful star going to New York (5)
'cheerful' is the definition.
(similar in meaning)
'star going to new york' is the wordplay.
'star' becomes 'sun' (Sun is an example).
'going to' is a charade indicator (letters next to each other).
'new york' becomes 'ny'.
(Other definitions for sunny that I've seen before include "Bright and cheerful", "Bright and warm", "Unclouded - cheerful", "Cheerful; fine (weather)", "(Of a person's disposition) always positive".)