Quiet river drowns Scotsman (5)
I believe the answer is:
piano
'quiet' is the definition.
(I've seen this before)
'river drowns scotsman' is the wordplay.
'river' becomes 'Po' (Italian river).
'drowns' is an insertion indicator (I've seen this in other clues).
'scotsman' becomes 'ian' (common Scottish name).
'po' placed around 'ian' is 'PIANO'.
(Other definitions for piano that I've seen before include "Musical instrument - play quietly" , "entertain with this softly" , "Soft, softly (music)" , "Keyboard instrument" , "Musical instrument - softly, please" .)