Bottomless River Plate's source (4)
I believe the answer is:
deep
'bottomless' is the definition.
(similar in meaning)
'river plate's source' is the wordplay.
'river' becomes 'dee' (River Dee).
'source' says to take the initial letters (source can mean a beginning or origin).
The first letter of 'plates' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Sunk low" , "Mysterious" , "Bottomless" , "profoundly" , "Extending far down" .)