Become firmer with one input returning (4-2)

I believe the answer is:

tone-up

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

'become firmer' is the definition.
I can't judge whether this definition defines the answer.

'with one input returning' is the wordplay.
'with' indicates putting letters inside.
'input' becomes 'put' (I am not sure about this - if you are sure you should believe this answer much more).
'returning' says the letters should be written in reverse.
'put' backwards is 'tup'.
'one' inserted inside 'tup' is 'TONE-UP'.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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.