Monstrous thing, beastly female -- horror! (9)
I believe the answer is:
nightmare
'horror' is the definition.
(I've seen this before)
'monstrous thing beastly female' is the wordplay.
'monstrous' indicates an anagram.
'beastly female' becomes 'mare' (I can't explain this - if you can you should believe this answer much more).
'thing' with letters rearranged gives 'night'.
'night'+'mare'='NIGHTMARE'
Can you help me to learn more?
(Other definitions for nightmare that I've seen before include "Horrifying experience" , "fiend" , "Frightening nocturnal fantasy" , "Awful experience" , "Terrifying experience" .)