Spooner's slice will astound participant in rally (11)
I believe the answer is:
shuttlecock
'rally' is the definition.
The definition and answer can be both related to competing as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for shuttlecock that I've seen before include "Item used in badminton" , "Essential bit of badminton equipment" , "Feathered cork in badminton" , "Object struck in badminton" , "The "ball" in badminton" .)