Spider-Man's inner skin layer (5)
I believe the answer is:
derma
'skin layer' is the definition.
(the derma or dermis is a layer of the skin)
'spider-man's inner' is the wordplay.
'inner' says the answer is hidden in the clue (I've seen 'in' mean this).
'DERMA' is hidden in the letters of 'spider man'.
(Other definitions for derma that I've seen before include "Deep vascular inner layer of the skin" , "Second protective layer" , "Corium, the inner layer of skin" , "Layer of living skin below the surface" , "Flesh" .)