Zone dealing with stopping motorists (4)
I believe the answer is:
area
'zone' is the definition.
(I've seen this before)
'dealing with stopping motorists' is the wordplay.
'dealing with' becomes 're' (regarding).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'motorists' becomes 'aa' (I've seen this in other clues).
're' placed into 'aa' is 'AREA'.
(Other definitions for area that I've seen before include "Extent of a two-dimensional surface" , "Extent of a 2-dimensional surface" , "environs" , "Length multiplied by breadth" , "field of study" .)