Tiny bit (4)
(Other definitions for iota that I've seen before include "Gr. letter; small amount" , "(Not one) little bit" , "Greek letter of little value" , "Tiny piece" , "Extremely small amount" .)
'tiny bit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for atom that I've seen before include "A tiny particle of matter with a big bang" , "Very small bit" , "little amount" , "Something very small perhaps" , "type of bomb" .)