Initial image of tiny atom shows a very small amount (4)
I believe the answer is:
iota
'a very small amount' is the definition.
(I've seen this before)
'initial image of tiny atom' is the wordplay.
'initial' says to take the initial letters.
The initial letters of 'image of tiny atom' is 'IOTA'.
'shows' is the link.
(Other definitions for iota that I've seen before include "Old letter" , "Tiny bit" , "Just a jot, from Greek alphabet" , "very small part" , "The Greek i (4)" .)