Only the first sixteen letters in one's head? (2,3)
I believe the answer is:
up top
'in one's head?' is the definition.
(up top can mean in one's mind)
I can't explain the rest of the clue.
(Other definitions for up top that I've seen before include "by way of intelligence" , "In respect of intelligence" , "in the brain" , "relating to intelligence" , "In the head (colloq.)" .)