Couple covering grass in competition (4,4)
I believe the answer is:
boat race
'competition' is the definition.
(I know that boat race is a type of race)
'couple covering grass' is the wordplay.
'couple' becomes 'brace' (brace can mean a pair of something).
'covering' means one lot of letters goes inside another.
'grass' becomes 'oat' (oat is a type of grass).
'brace' placed around 'oat' is 'BOAT-RACE'.
'in' is the link.
(Other definitions for boat race that I've seen before include "Oxbridge contest" , "Sports event" , "Cockney's face" , "varsity match" , "Cockney has to face" .)