Get down to it and write in for the job personally (5,7)

I believe the answer is:

apply oneself

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

'get down to it and write in' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'the job personally' is the wordplay.
'the job' becomes 'apply' (I can't explain this - if you can you should give a lot more credence to this answer).
'personally' becomes 'oneself' (I've seen this before).
'apply'+'oneself'='APPLY ONESELF'

'for' acts as a link.

Can you help me to learn more?

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.