Around noon, what Romans once put on vehicle (5)
I believe the answer is:
tonga
'vehicle' is the definition.
(I have seen 'Light vehicle' mean 'tonga' so perhaps 'vehicle' could also mean 'tonga')
'around noon what romans once put on' is the wordplay.
'around' means one lot of letters goes inside another.
'noon' becomes 'n' (abbreviation for noon).
'what romans once put on' becomes 'toga' (Romans wore togas).
'n' put into 'toga' is 'TONGA'.
(Other definitions for tonga that I've seen before include "Island nation" , "Pacific Ocean monarchy" , "transport" , "group of Pacific islands" , "Do a bad tango in the Pacific archipelago" .)