Row about double-parking a sort of truck (6)
I believe the answer is:
tipper
'a sort of truck' is the definition.
(I know that tipper is a type of motortruck)
'row about double-parking' is the wordplay.
'row' becomes 'tier' (a tier is a row of things).
'about' is an insertion indicator.
'double' shows that letters should be duplicated.
'parking' becomes 'p' (abbreviation on road signs).
'p' duplicated is 'pp'.
'tier' placed around 'pp' is 'TIPPER'.
(Other definitions for tipper that I've seen before include "One who is wont to give gratuities" , "One who thankfully pays extra" , "One giving a gratuity" , "Person who gives gratuity" , "One giving gratuities - truck" .)