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 soupcon that I've seen before include "A vary small quantity, just a trace - from France" , "very little" , "Smidgen" , "Tiny quantity" , "suspicion" .)
(Other definitions for whisker that I've seen before include "Coarse hair" , "Hair on cat's face" , "Bristle; small amount" , "Sensory hair of a cat" , "Hair's breadth" .)