Competes in race that’s most laborious (8)
I believe the answer is:
heaviest
'most laborious' is the definition.
('heavy' can be a synonym of 'laborious')
'competes in race' is the wordplay.
'competes' becomes 'vies' ('vie' can be a synonym of 'compete').
'in' means one lot of letters goes inside another.
'race' becomes 'heat' (I've seen this before).
'vies' put into 'heat' is 'HEAVIEST'.
'that's' acts as a link.
(Other definitions for heaviest that I've seen before include "Weighing the most" , "Weightiest" , "Of comparatively great physical weight or density" , "Most difficult to shift" , "Having the greatest weight" .)