Keep in your head (4)
I believe the answer is:
mind
'keep' is the definition.
(thesaurus)
'in your head' is the wordplay.
I cannot really understand how this works, but
'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mind that I've seen before include "Take care of; intellect" , "Object to; take care of" , "Look after" , "Consciousness" , "Intelligence" .)