Bottomless river associated with Parana's source (4)
I believe the answer is:
deep
'bottomless' is the definition.
(similar in meaning)
'river associated with parana's source' is the wordplay.
'river' becomes 'dee' (River Dee).
'associated with' says to put letters next to each other.
'source' suggests taking the first letters (I've seen 'sources' mean this).
The initial letter of 'parana' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Profound - cunning - sea - bass" , "Sunk low" , "Bottomless" , "Cavernous" , "Intense; low (in pitch)" .)