Penny cut off sound (4)
I believe the answer is:
plop
'sound' is the definition.
(plop is a kind of sound)
'penny cut off' is the wordplay.
'penny' becomes 'p'.
'cut off' becomes 'lop' (I've seen this before).
'p'+'lop'='PLOP'
(Other definitions for plop that I've seen before include "Sound of a stone landing in water" , "Sound of object falling into water without a splash" , "Report making little splash" , "Noise of a small object dropping into water" , "Sound of a stone hitting water" .)