An avenger spies English guys returning (7)
I believe the answer is:
nemesis
'an avenger' is the definition.
(thesaurus)
'spies english guys returning' is the wordplay.
'spies' becomes 'sis' (I can't justify this - if you can you should give a lot more credence to this answer).
'english' becomes 'e' (abbreviation).
'guys' becomes 'men' (guy can mean a man).
'returning' says the letters should be written backwards.
'sis'+'e'+'men'='sisemen'
'sisemen' reversed gives 'NEMESIS'.
Can you help me to learn more?
(Other definitions for nemesis that I've seen before include "Avenging deity" , "curse" , "Bane" , "Agent of (someone's) downfall" , "Greek goddess - opponent who cannot be beaten" .)