One loudly interrupting sister for a joke (2,3)
I believe the answer is:
in fun
'for a joke' is the definition.
(I've seen this in another clue)
'one loudly interrupting sister' is the wordplay.
'one' becomes 'i' (Roman numeral).
'loudly' becomes 'f' (short for forte - 'loud' in music).
'interrupting' indicates putting letters inside.
'sister' becomes 'nun' (nuns are referred to as sister).
'f' going into 'nun' is 'nfun'.
'i'+'nfun'='IN FUN'
(Other definitions for in fun that I've seen before include "to raise a smile?" , "for a laugh" , "Not intended seriously" , "As a joke - not seriously" .)