River quarter's sound (5)
I believe the answer is:
forth
'river' is the definition.
(Scottish river)
'quarter's sound' is the wordplay.
'quarter' becomes 'fourth' (a quarter is a fourth of something).
'sound' indicates a 'sounds like' (homophone) clue.
'fourth' is a homophone of 'FORTH'.
(Other definitions for forth that I've seen before include "Onward; the Firth of - -" , "(Go) away" , "Onward direction" , "River and estuary in eastern Scotland" , "Water" .)