Bishop with a single part of skeleton (4)
I believe the answer is:
bone
'part of skeleton' is the definition.
(I've seen this before)
'bishop with a single' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'with' is a charade indicator (letters next to each other).
'a single' becomes 'one' (a single thing - only one).
'b'+'one'='BONE'
(Other definitions for bone that I've seen before include "Carpal or tarsal, for example" , "Skeleton substance" , "Part of skeleton" , "Skeleton material" , "#Close to it is risque" .)