Very small amount (4)
'very small amount' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iota that I've seen before include "Grain" , "Tiny or scarcely detectable amount" , "Just a jot, like Greek letter" , "Gk. letter; tiny amount" , "Gr. letter; small amount" .)
(Other definitions for whisker that I've seen before include "Hair on cat's face" , "Long stiff hair, of a cat say" , "Bristle; small amount" , "Long facial hair" , "narrow margin" .)