Her sporting career is going rapidly downhill (5)
I believe the answer is:
skier
'downhill' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for skier that I've seen before include "Winter sportsman" , "He glides over snow, perhaps in sport" , "he's possibly going downhill quickly" , "One on the piste" , "Sport exponent" .)