Celebration that is involved in breaking fast (6)
I believe the answer is:
fiesta
'celebration' is the definition.
(fiesta is a kind of celebration)
'that is involved in breaking fast' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'involved in' indicates putting letters inside.
'breaking' indicates an anagram.
'fast' is an anagram of 'fsta'.
'ie' placed within 'fsta' is 'FIESTA'.
(Other definitions for fiesta that I've seen before include "Elaborate public party, in Spain perhaps" , "A Spanish public celebration" , "Saint's day; carnival" , "Holiday; carnival" , "A public holiday in Barcelona" .)