Queen intercepts motorists going over field (4)
I believe the answer is:
area
'field' is the definition.
(synonyms)
'queen intercepts motorists going over' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'intercepts' is an insertion indicator (I've seen this in other clues).
'motorists' becomes 'aa' (I've seen this in other clues).
'going over' shows that the letters should be reversed in order.
'er' inserted within 'aa' is 'aera'.
'aera' written backwards gives 'AREA'.
(Other definitions for area that I've seen before include "The measure of a surface" , "Space for a specific purpose, say picnic" , "Space occupied by something" , "District; locale" , "Geographical region" .)