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Most of the advice given by forecasters goes over the head of one on site (6)

Ross

I believe the answer is:

helmet

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'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" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).
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