Discovery includes English monster (5)
I believe the answer is:
fiend
'monster' is the definition.
(I know that monster can be written as fiend)
'discovery includes english' is the wordplay.
'discovery' becomes 'find' (synonyms).
'includes' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'find' going around 'e' is 'FIEND'.
(Other definitions for fiend that I've seen before include "Devil" , "Devilish person" , "Operatic super-soprano" , "Wicked and inhuman person" , "Evil supernatural being" .)