Long lead taken from back of Macintosh (4)
I believe the answer is:
itch
'long' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for itch that I've seen before include "source of irritation" , "Slight irritation" , "Burning" , "Impatient desire (colloq.)" , "burn" .)