Plot as group of drivers entering racing event (5)
I believe the answer is:
tract
'plot' is the definition.
(land)
'group of drivers entering racing event' is the wordplay.
'group of drivers' becomes 'RAC' (Royal Automobile Club).
'entering' means one lot of letters goes inside another.
'racing event' becomes 'TT' (Isle of Man Tourist Trophy races).
'rac' put inside 'tt' is 'TRACT'.
'as' acts as a link.
(Other definitions for tract that I've seen before include "religious work in > area" , "lot of ground?" , "Anatomical structure" , "Stretch of land or religious pamphlet" , "Extended area of land" .)