Emulsion thrown over top of informer's car (9)
I believe the answer is:
limousine
'car' is the definition.
(limousine is a kind of car)
'emulsion thrown over top of informer's' is the wordplay.
'thrown' is an anagram indicator.
'over' means one lot of letters goes inside another.
'top of' says to take the initial letters.
The first letter of 'informers' is 'i'.
'emulsion' is an anagram of 'limousne'.
'limousne' placed around 'i' is 'LIMOUSINE'.
(Other definitions for limousine that I've seen before include "Luxurious car (with a chauffeur?)" , "big car" , "Form of transport" , "Luxury car" , "Large luxurious car" .)