Smallest particle of an element (4)
I believe the answer is:
atom
This clue is a double definition.
'smallest particle' is the first definition.
(I've seen this before)
'an element' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for atom that I've seen before include "Smallest particle in an element able to take part in a chemical reaction" , "It was once thought to be smallest particle of matter" , "Very small object or amount" , "Part of molecule" , "Basic unit of a chemical element" .)