Itsy-bitsy bit (4)
I believe the answer is:
atom
'itsy-bitsy bit' 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 "Very small object or amount" , "A wee bit" , "A tiny particle of matter with a big bang" , "Tiny bit" , "Particle of matter now divided" .)