River meets quiet sea (4)
I believe the answer is:
deep
'sea' is the definition.
(thesaurus)
'river meets quiet' is the wordplay.
'river' becomes 'dee' (River Dee).
'meets' is a charade indicator (letters next to each other).
'quiet' becomes 'p' (music symbol for 'piano').
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Sunk low" , "Profound - engrossed" , "Mysterious" , "dangerous for swimming?" , "Intense; low (in pitch)" .)