Rubber ring not punctured by actor – one's always ready and available (6-2-3-4)

I believe the answer is:

johnny-on-the-spot

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

'one's always ready and available' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'rubber ring not punctured by actor' is the wordplay.
'rubber' becomes 'johnny' (I can't explain this - if you can you should give a lot more credence to this answer).
'ring' becomes 'O' (letter resembling a ring).
'punctured by' indicates putting letters inside (inserted letters puncture into the word).
'actor' becomes 'thesp' (short for thespian, term for an actor).
'not' enclosing 'thesp' is 'nthespot'.
'johnny'+'o'+'nthespot'='JOHNNY-ON-THE-SPOT'

Can you help me to learn more?

I've seen this clue in The Independent.

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.