It controls the central nervous system (5)
I believe the answer is:
brain
'it controls the central nervous system' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brain that I've seen before include "to get hit over the head" , "Seat of the intellect" , "Haemorrhaging of talent" , "Encephalon" , "Mind" .)