Letter covered in skin and bone (5)
I believe the answer is:
femur
'bone' is the definition.
(I know that femur is a bone)
'letter covered in skin' is the wordplay.
'letter' becomes 'em' (I've seen this in other clues).
'covered in' means one lot of letters goes inside another.
'skin' becomes 'fur' (fur is a kind of skin).
'em' put within 'fur' is 'FEMUR'.
'and' acts as a link.
(Other definitions for femur that I've seen before include "Thighbone" , "Thigh bone" , "Large bone" , "Part of leg" , "Longest bone in the human body" .)