Mythical creature of unusual nature with colt's head (7)
I believe the answer is:
centaur
'mythical creature' is the definition.
(centaur is a kind of mythical creature)
'unusual nature with colt's head' is the wordplay.
'unusual' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'head' suggests taking the first letters.
The first letter of 'colts' is 'c'.
'nature' with letters rearranged gives 'entaur'.
'entaur' put after 'c' is 'CENTAUR'.
'of' acts as a link.
(Other definitions for centaur that I've seen before include "Mythical hybrid" , "Mythical horseman!" , "'Mythical creature, half man, half horse (7)'" , "Mythical creature with the head of a man and the body of a horse" , "Mythical creature, half-man, half-horse" .)