Letter before kappa (4)
I believe the answer is:
iota
'letter before kappa' 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 "'Very small amount, jot (4)'" , "Greek equivalent of the letter I" , "Just a jot, like Greek letter" , "Smidgeon" , "Extremely small amount" .)