Creature right inside mist (4)
I believe the answer is:
frog
'creature' is the definition.
(frog is a kind of creature)
'right inside mist' is the wordplay.
'right' becomes 'r' (abbreviation).
'inside' indicates putting letters inside.
'mist' becomes 'fog' (synonyms).
'r' inserted into 'fog' is 'FROG'.
(Other definitions for frog that I've seen before include "Tailless stout-bodied amphibian" , "Croak in the throat?" , "in grass?" , "Adult tadpole" , "Common amphibian" .)