Einstein hit hard on head (5)
I believe the answer is:
brain
This might be a double definition.
'einstein' is the first definition.
(I've seen this in another clue)
'hit hard on head' is the second definition.
'brain' can be an answer for 'hit' (braining is a kind of hitting). I'm not sure about the 'hard on head' bit.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Whack on the head" , "Hit hard; intellect" , "Haemorrhaging of talent" , "The grey matter" , "Intelligent person" .)