Dud, quiet swimmer, added zest (5,4)
I believe the answer is:
lemon peel
'zest' is the definition.
(I've seen this before)
'dud quiet swimmer added' is the wordplay.
'dud' becomes 'lemon' (I've seen this in another clue).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'swimmer' becomes 'eel' (eel is a type of fish).
'added' says to put letters next to each other.
'lemon'+'p'+'eel'='LEMON PEEL'
(Other definitions for lemon peel that I've seen before include "twist" , "Source of zest" .)