Mist surrounding river amphibian (4)
I believe the answer is:
frog
'amphibian' is the definition.
(frog is a kind of amphibian)
'mist surrounding river' is the wordplay.
'mist' becomes 'fog' ('fog' can be a synonym of 'mist').
'surrounding' means one lot of letters goes inside another.
'river' becomes 'r'.
'fog' placed around 'r' is 'FROG'.
(Other definitions for frog that I've seen before include "One of Ranidae" , "Leaping amphibian" , "fastener" , "Amphibious neighbour of 12?" , "Batrachian" .)