Training after meal firstly causing injury (7)

I believe the answer is:

maiming

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

'injury' is the definition.
(maiming is a kind of injury)

'training after meal firstly' is the wordplay.
'training' becomes 'aiming' ('aim' can be a synonym of 'train').
'after' says to put letters next to each other.
'meal firstly' becomes 'm' (I can't explain this - if you can you should give a lot more credence to this answer).
'aiming' after 'm' is 'MAIMING'.

'causing' is the link.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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.