Volunteers mutant superhero to be one overseeing duty (6)
I believe the answer is:
tax-man
'overseeing duty' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'volunteers mutant superhero to be one' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'mutant superhero' becomes 'xm' (I can't justify this - if you can you should believe this answer much more).
'to be one' becomes 'an' (I've seen this in other clues).
'ta'+'xm'+'an'='TAX-MAN'
(Other definitions for tax-man that I've seen before include "HMRC employee" , "HMRC official" , "Male official who collects government revenue" .)