What an Impossible Burger lacks
I believe the answer is:
meat
'what an impossible burger lacks' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for meat that I've seen before include "Flesh for eating" , "Food for carnivores" , "grub, say" , "17 [ESSENCE]" , "The flesh of animals as food" .)