Exert to the utmost in race (6)
I believe the answer is:
strain
This clue is a double definition.
'exert to the utmost' is the first definition.
(I've seen this before)
'race' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for strain that I've seen before include "Violent effort" , "Drain off" , "song" , "Distinct breed; use a seive" , "In arts (anag)" .)