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' means one lot of letters go next to another.
'the fourth of july' becomes 'y' (fourth letter of the word 'July').
'can be' is an anagram indicator.
'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" .)