Take care of brain (4)
I believe the answer is:
mind
I believe this clue is a double definition.
'take care' is the first definition.
(I've seen this before)
'brain' is the second definition.
(I know that brain can be written as mind)
'of' is the link.
(Other definitions for mind that I've seen before include "Object to; take care of" , "Watch - intellect" , "Look after" , "Brain" , "'Out of sight, out of ....'" .)