Maiden opposing key monster in myth (9)
I believe the answer is:
manticore
'monster in myth' is the definition.
(mythical creature)
'maiden opposing key' is the wordplay.
'maiden' becomes 'm' (cricket abbreviation).
'opposing' becomes 'anti' (prefix meaning 'opposing').
'key' becomes 'core' (both can mean majorly important).
'm'+'anti'+'core'='MANTICORE'
(Other definitions for manticore that I've seen before include "Mythical stinging beast" , "Fabulous animal" .)