Mythological fire-breathing creature (6)
I believe the answer is:
dragon
'mythological fire-breathing creature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dragon that I've seen before include "Fierce woman" , "St George is said to have killed one" , "Reptilian monster" , "Fire-breathing, winged mythical monster" , "Mythical fire-breather" .)