Amphibian in river enveloped in thick mist (4)
I believe the answer is:
frog
'amphibian' is the definition.
(frog is a kind of amphibian)
'river enveloped in thick mist' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'enveloped in' is an insertion indicator.
'thick mist' becomes 'fog' (I've seen this before).
'r' going inside 'fog' is 'FROG'.
'in' acts as a link.
(Other definitions for frog that I've seen before include "Croaker?" , "fastener" , "throat problem" , "in grass?" , "Croaking amphibian" .)