On the fourth of July, nuns can be cheerful (5)
I believe the answer is:
sunny
'cheerful' is the definition.
(similar in meaning)
'on the fourth of july nuns can be' is the wordplay.
'on' is a charade indicator (letters next to each other).
'the fourth of july' becomes 'y' (fourth letter of the word 'July').
'can be' indicates anagramming the letters.
'nuns' anagrammed gives 'sunn'.
'y' after 'sunn' is 'SUNNY'.
(Other definitions for sunny that I've seen before include "cheerful sound" , "Unclouded - cheerful" , "(Of a person's disposition) always positive" , "Bright and warm" , "Genial, cheerful" .)