Break up fourth of July celebration (5)
I believe the answer is:
party
'celebration' is the definition.
(thesaurus)
'break up fourth of july' is the wordplay.
'break up' becomes 'part' ('part' can be a synonym of 'break up').
'fourth of july' becomes 'y' (fourth letter of the word 'July').
'part'+'y'='PARTY'
(Other definitions for party that I've seen before include "MP's faction" , "Social gathering; person" , "Social occasion - political group" , "eg Labour, Conservative" , "Celebratory social gathering" .)