Motorists getting on in district (4)
I believe the answer is:
area
'district' is the definition.
(a district is a geographical area)
'motorists getting on' is the wordplay.
'motorists' becomes 'AA' (Automobile Association).
'getting' is an insertion indicator.
'on' becomes 're' (regarding).
'aa' going around 're' is 'AREA'.
'in' acts as a link.
(Other definitions for area that I've seen before include "Speaking formally to (someone)" , "District; locale" , "field of study" , "Length multiplied by breadth" , "Expanse of a surface" .)