Mist hides head of rare animal (4)
I believe the answer is:
frog
'animal' is the definition.
(frog is a kind of animal)
'mist hides head of rare' is the wordplay.
'mist' becomes 'fog' (synonyms).
'hides' means one lot of letters goes inside another.
'head of' says to take the initial letters.
The first letter of 'rare' is 'r'.
'fog' enclosing 'r' is 'FROG'.
(Other definitions for frog that I've seen before include "Croak in the throat?" , "Grown-up tadpole" , "Amphibious neighbour of 12?" , "An amphibian; a track plate" , "Common amphibian" .)