What superhero might have on the outside? (9)
I believe the answer is:
underwear
'what superhero might have on the outside?' is the definition.
(Superman and others!)
This is all the clue.
(Other definitions for underwear that I've seen before include "Hidden clothing" , "Smalls,shorts,lingerie etc" , "Drawers" , "Possibly slips" , "Lingerie" .)