Sent nine off to first sporting obstacle (6,3)

I believe the answer is:

tennis net

I'm a little stuck... Click here to teach me more about this clue!

'sporting obstacle' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'sent nine off to first' is the wordplay.
'off' indicates anagramming the letters.
'to first' becomes 't' (1st letter of 'to').
'sent'+'nine'='sentnine'
'sentnine' anagrammed gives 'tennisne'.
'tennisne'+'t'='TENNIS NET'

Can you help me to learn more?

I've seen this clue in The Daily Express.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.