Training period of rifle carrying finished (9)

I believe the answer is:

pupillage

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

'training period' is the definition.
The definition suggests a singular noun which matches the answer.

'rifle carrying finished' is the wordplay.
'rifle' becomes 'pillage' (both can mean to rob a place).
'carrying' indicates putting letters inside.
'finished' becomes 'up' (eg 'time's up' means the time is finished).
'pillage' enclosing 'up' is 'PUPILLAGE'.

'of' 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.