Head: might that be screwed on? (3)
I believe the answer is:
nut
'head' is the definition.
(informal term for somebody's head)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nut that I've seen before include "One securing bolt" , "Cashew or hazel" , "organized teachers" , "buff" , "Fan" .)