I loudly interrupt a good woman, but not seriously (2,3)
I believe the answer is:
in fun
'not seriously' is the definition.
(I've seen this in another clue)
'i loudly interrupt a good woman' is the wordplay.
'loudly' becomes 'f' (short for forte - 'loud' in music).
'interrupt' means one lot of letters goes inside another.
'a good woman' becomes 'nun'.
'f' placed within 'nun' is 'nfun'.
'i'+'nfun'='IN FUN'
'but' acts as a link.
(Other definitions for in fun that I've seen before include "for a laugh" , "As a joke - not seriously" , "to raise a smile?" , "Not intended seriously" .)