A very small part in any body (7)
I believe the answer is:
anatomy
'body' is the definition.
(I know that anatomy is a type of body)
'a very small part in any' is the wordplay.
'a very small part' becomes 'atom' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'atom' inserted into 'any' is 'ANATOMY'.
Can you help me to learn more?
(Other definitions for anatomy that I've seen before include "Scientific study of bodily structure" , "Science of bodily structure" , "Branch of biology" , "The scientific study of bodily structures" , "Study of the structure of animals" .)