Swear to overwhelm English member of superhero squad (7)
I believe the answer is:
avenger
'member of superhero squad' is the definition.
(member of The Avengers in Marvel comics)
'swear to overwhelm english' is the wordplay.
'swear' becomes 'aver' (synonyms).
'to overwhelm' indicates putting letters inside.
'english' becomes 'eng' (abbreviation).
'aver' placed around 'eng' is 'AVENGER'.
(Other definitions for avenger that I've seen before include "One settles score" , "One bent on retaliation." , "One inflicting retribution" , "One wanting retribution" , "One who gets even" .)