Monster - English writer almost entirely upset (5)
I believe the answer is:
hydra
'monster' is the definition.
(Hydra is an example)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hydra that I've seen before include "Femme fatale of legend" , "Water monster" , "Mythic monster with nine heads" , "Proliferating-head monster" , "Persistent trouble" .)