Crossword Genius

Oppose petition, swapping sides (4)

Ross

I believe the answer is:

play

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'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" .)

I've seen this clue in The Telegraph.
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