Scotsman in river played quietly (5)
I believe the answer is:
piano
'quietly' is the definition.
(synonyms)
'scotsman in river' is the wordplay.
'scotsman' becomes 'ian' (common Scottish name).
'in' is an insertion indicator.
'river' becomes 'Po' (Italian river).
'ian' put within 'po' is 'PIANO'.
'played' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for piano that I've seen before include "Quiet (music)" , "quiet passage" , "key holder" , "p (mus.)" , "soft passage" .)