Very smallest part of anatomy? (4)
I believe the answer is:
atom
'very smallest part' is the definition.
'of anatomy?' is the wordplay.
'of' indicates a hidden word.
'ATOM' can be found hidden inside 'anatomy'.
(Other definitions for atom that I've seen before include "A mighty physical particle or bomb" , "A mighty particle, can be explosive" , "tiny thing" , "A small but mighty particle" , "Particle; molecule" .)