Make an effort in race (6)
I believe the answer is:
strain
I believe this clue is a double definition.
'make an effort' is the first definition.
(I've seen this before)
'race' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for strain that I've seen before include "Overwork - melody" , "Drain off" , "Strive; ancestry" , "Tension and exertion" , "tax" .)