Most of the advice given by forecasters goes over the head of one on site (6)
I believe the answer is:
helmet
'head of one on site' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for helmet that I've seen before include "Police hat" , "Hard hat" , "Safety hat" , "personal protection" , "Protective head-covering for armour or motor cyclist" .)