Carol, with mashed potato, perhaps making a fuss (4,3,5)
I believe the answer is:
song and dance
'a fuss' is the definition.
(I have seen 'Unnecessary fuss' mean 'song and dance' so perhaps 'fuss' could also mean 'song and dance')
'carol with mashed potato perhaps' is the wordplay.
'carol' becomes 'song' (I've seen this in another clue).
'with' becomes 'and'.
'mashed potato perhaps' becomes 'dance' (I can't explain this - if you can you should believe this answer much more).
'song'+'and'+'dance'='SONG-AND-DANCE'
'making' acts as a link.
Can you help me to learn more?
(Other definitions for song and dance that I've seen before include "commotion" , "Kerfuffle" , "Unnecessary fuss" , "Routine" , "Silly fuss; some entertainment" .)