Unravel mystery of sea creature existing in freshwater river? (6)
I believe the answer is:
decode
'unravel' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for decode that I've seen before include "Find hidden meaning" , "Do translation" , "Decipher, decrypt" , "Penetrate or solve encrypted message" , "Break a cipher" .)