Hidden stem by a river (6)
I believe the answer is:
arcane
'hidden' is the definition.
(thesaurus)
'stem by a river' is the wordplay.
'stem' becomes 'cane' (cane is a kind of stem).
'by' says to put letters next to each other.
'river' becomes 'r'.
'a'+'r'='ar'
'cane' put after 'ar' is 'ARCANE'.
(Other definitions for arcane that I've seen before include "Mysterious like a crane" , "Mysterious, secret or obscure" , "Secret, hidden" , "Ace ran, being mysterious" , "Recondite" .)