They’re in fast car describing rubbish acceleration (10)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

GTI(abbrev. for “Grand Tourer Injection” from “Gran Turismo Iniezione”, denoting a high performance /fast car with a fuel-injection engine) containing ( describing ) [ LITTER(rubbish left lying in an open or public place) + A(symbol for “acceleration” in physics) ]. Defn: …/they are members of the fashionable set.