Jellyfish's lack (5)
I believe the answer is:
brain
'jellyfish's lack' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for brain that I've seen before include "Encephalon" , "Whack on the head" , "Control centre of nervous system" , "Organ of the intellect" , "It's in the skull" .)