Indispensable member of locksmith's staff? (3,3)

I believe the answer is:

key man

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

'indispensable member' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'locksmith's staff?' is the wordplay.
'locksmith's' becomes 'key' (I can't explain this - if you can you should give a lot more credence to this answer).
'staff?' becomes 'man' (as in staffing or manning a particular place).
'key'+'man'='KEY MAN'

'of' is the link.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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.