Scientist returned vehicle to policeman (5)
I believe the answer is:
dirac
'scientist' is the definition.
(I've seen this before)
'returned vehicle to policeman' is the wordplay.
'returned' shows that the letters should be reversed in order.
'vehicle' becomes 'car' (car is a kind of vehicle).
'to' means one lot of letters go next to another (I've seen this in other clues).
'policeman' becomes 'di' (abbreviation for Detective Inspector).
'car' reversed gives 'rac'.
'rac' after 'di' is 'DIRAC'.
(Other definitions for dirac that I've seen before include "Nobel Prize winner" , "English physicist" , "constant originator" , "Scientist" , "Paul -, physicist; I'd arc (anag.)" .)