Beginning to cause fuss over ending (4)
I believe the answer is:
coda
'ending' is the definition.
(I have seen 'Musical ending ' mean 'coda' so perhaps 'ending' could also mean 'coda')
'beginning to cause fuss over' is the wordplay.
'beginning to' says to take the initial letters.
'fuss' becomes 'ado' ('ado' can be a synonym of 'fuss').
'over' says the letters should be written in reverse.
The first letter of 'cause' is 'c'.
'ado' in reverse letter order is 'oda'.
'c'+'oda'='CODA'
(Other definitions for coda that I've seen before include "End piece written for a musical work" , "Closing part of a musical composition" , "final piece of work" , "Tailpiece" , "Closing passage of a piece of music" .)