Crossword Genius

Only the first sixteen letters in one's head? (2,3)

Ross

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.)" .)

I've seen this clue in The Times.
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