River gets clogged up in sound (4)
I believe the answer is:
styx
'river' is the definition.
(mythical river)
'gets clogged up in sound' is the wordplay.
'gets clogged up' becomes 'sticks' (eg gets stuck in a drain).
'in sound' indicates a 'sounds like' (homophone) clue.
'sticks' is a homophone of 'STYX'.
(Other definitions for styx that I've seen before include "River around Hades" , "Hellish current" , "'River of Hate' in Hades" , "River Charon rowed on" , "Underworld river; sounds like remote area" .)