Head screwed on? (3)
I believe the answer is:
nut
'head' is the definition.
(informal term for somebody's head)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nut that I've seen before include "Brazil, for instance" , "Hard-skinned fruit" , "eg Almond, brazil" , "Fan" , "Crank" .)