Jet, one about to damage a police vehicle (5,5)
I believe the answer is:
black maria
'police vehicle' is the definition.
(informal term for a vehicle transporting prisoners)
'jet one about to damage a' is the wordplay.
'jet' becomes 'black' (similar in meaning).
'one' becomes 'i' (Roman numeral).
'about' says to put letters next to each other.
'to damage' becomes 'mar' (to mar is to spoil or damage).
'i' after 'mar' is 'mari'.
'black'+'mari'+'a'='BLACK MARIA'
(Other definitions for black maria that I've seen before include "Police prisoners' transport" , "Nick vehicle" , "Old police van for transporting prisoners" , "Van with criminals" , "vehicle for criminals" .)