Bohr study (4)
I believe the answer is:
atom
'bohr study' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for atom that I've seen before include "Tiny particle, causes mighty explosion" , "A wee bit" , "Smallest amount" , "Smallest particle in an element able to take part in a chemical reaction" , "Piece of matter in moat" .)