Mysterious river ending in swamp (4)
I believe the answer is:
deep
'mysterious' is the definition.
(thesaurus)
'river ending in swamp' is the wordplay.
'river' becomes 'dee' (River Dee**).
'ending in' indicates one should take the final letters.
The final letter of 'swamp' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "late" , "Bottomless" , "Profound - cunning - sea - bass" , "Extending far down" , "dangerous for swimming?" .)