An urge to scratch (4)
I believe the answer is:
itch
'an urge to scratch' 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 "Tickle; urge" , "minor irritant" , "Seven-yearly desire" , "It needs to be scratched" , "Longing; anag. of 19" .)