A plot in which race is involved (4)
I believe the answer is:
acre
'a plot' is the definition.
(I've seen this in another clue)
'race is involved' is the wordplay.
'is involved' is an anagram indicator (involved can mean confusing or complex).
'race' anagrammed gives 'ACRE'.
'in which' is the link.
(Other definitions for acre that I've seen before include "Middle East port" , "Race (anag.)" , "Mediterranean port" , "amount of land" , "Imperial unit of area" .)