Iodine reaction
I believe the answer is:
sting
'iodine reaction' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sting that I've seen before include "English musician, b. 1951 -- a sudden pain" , "The pain caused by bee or nettle" , "Entrapment operation; sharp pain" , "18, sharp pain" , "Bee/wasp wound" .)