Rebecca lost a silly sports day contest? (8,4)
I believe the answer is:
obstacle race
'sports day contest?' is the definition.
(a school sports day might feature an obstacle race)
'rebecca lost a silly' is the wordplay.
'silly' indicates an anagram.
'rebecca'+'lost'+'a'='rebeccalosta'
'rebeccalosta' anagrammed gives 'OBSTACLE RACE'.
(Other definitions for obstacle race that I've seen before include "Sports event - Carol, best ace (anag)" , "novelty event" , "Event with assorted hindrances" , "Contest with delaying features" , "sporting event" .)