When a vehicle gets past, this protects the head (5)
I believe the answer is:
busby
'when a vehicle gets past this protects the head' is the definition.
The definition suggests a singular noun which matches the answer.
'past this protects the head' is the wordplay.
'past' becomes 'by' ('by' can be a synonym of 'past').
'this protects' is an insertion indicator.
'the head' becomes 'usb' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' enclosing 'usb' is 'BUSBY'.
Can you help me to learn more?
(Other definitions for busby that I've seen before include "Guardsman's helmet" , "Hussar's hat" , "Military fur hat" , "Guardsman's fur hat" , "Bearskin cap (colloq.)" .)