Einstein hit on head (5)
I believe the answer is:
brain
This clue is a triple definition!
'einstein' is the first definition.
(I've seen this in another clue)
'hit' is the second definition.
(braining is a kind of hitting)
'on' is a link.
'head' is the third definition.
(I know that head can be written as brain)
(Other definitions for brain that I've seen before include "Control centre of nervous system" , "Hit hard; intellect" , "Encephalon" , "Seat of the intellect" , "It controls the central nervous system" .)