Oppose petition, swapping sides (4)
I believe the answer is:
play
'oppose' is the definition.
The definition and answer can be both related to competing as well as being verbs in their base form.
Perhaps you can see an association between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for play that I've seen before include "Dramatic work" , "Recreation" , "Make sport" , "stage production" , "Theatrical performance" .)