Superhero's accessory conceals an appetizer (6)
I believe the answer is:
canape
'appetizer' is the definition.
(canape is a kind of appetizer)
'superhero's accessory conceals an' is the wordplay.
'superhero's accessory' becomes 'cape' (some superheroes wear capes).
'conceals' is an insertion indicator.
'cape' placed around 'an' is 'CANAPE'.
(Other definitions for canape that I've seen before include "Small savoury on toast or bread" , "Finger food for parties" , "Savoury appetiser, often accompanied by Champagne" , "Small appetising savoury" , "Savoury mouthful" .)