Scottish river, quiet and extending far down (4)
I believe the answer is:
deep
'extending far down' is the definition.
(I've seen this before)
'scottish river quiet' is the wordplay.
'scottish river' becomes 'dee' (I've seen this before**).
'quiet' becomes 'p' (music symbol for 'piano').
'dee'+'p'='DEEP'
'and' is the link.
(Other definitions for deep that I've seen before include "Cavernous" , "Mysterious" , "(Of sound) low in pitch" , "Main" , "Intense -- extreme" .)