Little bit of anatomy (4)
I believe the answer is:
atom
'little bit' is the definition.
(tiny particle)
'of anatomy' is the wordplay.
'of' says the answer is hidden in the clue.
'ATOM' is hidden in the letters of 'anatomy'.
(Other definitions for atom that I've seen before include "Very small object or amount" , "The tiniest particle" , "Nucleus surrounded" , "Something very small perhaps" , "tiny thing" .)