Poison ivy symptom (4)
I believe the answer is:
'poison ivy symptom' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for itch that I've seen before include "Something that needs a scratch" , "Irritating feeling on the skin" , "cause irritation" , "It needs to be scratched" , "Tickle; urge" .)