Tiny part in any field of study (7)
I believe the answer is:
anatomy
'field of study' is the definition.
(anatomy is a kind of field of study)
'tiny part in any' is the wordplay.
'tiny part' becomes 'atom' (I have seen 'Part of molecule ' mean 'atom' so perhaps 'part' could also mean 'atom'. I am not sure about the 'tiny' bit.).
'in' indicates putting letters inside.
'atom' placed into 'any' is 'ANATOMY'.
Can you help me to learn more?
(Other definitions for anatomy that I've seen before include "Branch of biology" , "(Science of) physical structure of animals or plants" , "Scientific study of bodily structure" , "Bodily structure of animal or plant" , "Study of bodies" .)