The smallest particle in any physical structure (7)
I believe the answer is:
anatomy
'physical structure' is the definition.
(anatomy is a kind of physical structure)
'the smallest particle in any' is the wordplay.
'the smallest particle' becomes 'atom' (I've seen this before).
'in' is an insertion indicator.
'atom' placed within 'any' is 'ANATOMY'.
(Other definitions for anatomy that I've seen before include "Study of (physical structure of) living things" , "Science of bodily structure" , "Scientific study of bodily structure" , "Study of bodies" , "(Science of) physical structure of animals or plants" .)