Amphibian, red top visible in thick mist (4)
I believe the answer is:
frog
'amphibian' is the definition.
(frog is a kind of amphibian)
'red top visible in thick mist' is the wordplay.
'top' suggests taking the first letters.
'visible in' is an insertion indicator.
'thick mist' becomes 'fog' (I've seen this before).
The first letter of 'red' is 'r'.
'r' going within 'fog' is 'FROG'.
(Other definitions for frog that I've seen before include "Croaker?" , "contribution to coughing?" , "Small creature" , "throat problem" , "Croak in the throat?" .)