Small bone needed for pincushion (5)
I believe the answer is:
incus
'small bone' is the definition.
(I know that incus is a type of small bone)
'needed for pincushion' is the wordplay.
'needed for' says the answer is hidden in the clue (the answer is needed to spell part of the clue).
'INCUS' is hidden in the letters of 'pincushion'.
(Other definitions for incus that I've seen before include "Bone in the middle ear" , "Middle-ear bone" , "Ear anvil bone" , "Anvil-shaped bone in ear" , "part of sound system" .)