Infinitesimal amount (4)
I believe the answer is:
iota
'infinitesimal amount' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iota that I've seen before include "Tiniest scrap" , "Ninthe letter of the Greek alphabet" , "very small part" , "(Not one) little bit" , "Grain" .)