Bottomless river gets quiet (4)
I believe the answer is:
deep
'bottomless' is the definition.
('deep' can be similar in meaning to 'bottomless')
'river gets quiet' is the wordplay.
'river' becomes 'dee' (River Dee**).
'gets' means one lot of letters go next to another.
'quiet' becomes 'p' (music symbol for 'piano').
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "down a long way" , "(Of sound) low in pitch" , "Main" , "hard to understand" , "Wise - difficult to understand" .)