Bladder mistakenly grafted with layer of skin (6)
I believe the answer is:
dermis
'layer of skin' is the definition.
(I've seen this in another clue)
'bladder mistakenly grafted with' is the wordplay.
'with' indicates a hidden word.
'DERMIS' can be found hidden inside 'bladder mistakenly grafted'.
(Other definitions for dermis that I've seen before include "The skin layer below the surface" , "Layer of skin beneath the outer" , "Layer of skin beneath the top one" , "True skin" , "Inner layer of skin" .)