Fire-breathing monster (6)
I believe the answer is:
dragon
'fire-breathing monster' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dragon that I've seen before include "Formidable woman" , "Fiery reptile or person" , "Legendary fire-breather" , "Mythological fire-breathing creature" , "Fire-breathing reptile, usually with wings" .)