Jellyfish defense (5)
I believe the answer is:
sting
'jellyfish defense' 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 "Sharp tingling pain" , "The pain caused by bee or nettle" , "Goad" , "18, sharp pain" , "Wound from insect" .)