Poison ivy reaction (4)
I believe the answer is:
itch
'poison ivy reaction' 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 "Prickle" , "Yen" , "Longing; anag. of 19" , "Hankering" , "source of irritation" .)