“Cheerful” is fair (5)
I believe the answer is:
sunny
I believe this is a double definition.
'cheerful' is the first definition.
('sunny' can be similar in meaning to 'cheerful')
'fair' is the second definition.
('sunny' can be similar in meaning to 'fair')
'is' acts as a link.
(Other definitions for sunny that I've seen before include "cheerful sound" , "(Of a person's disposition) always positive" , "Cheerful - cloudless" , "Unclouded - cheerful" , "Bright and cheerful" .)