Maths expert's lager tab is recalculated (10)

I believe the answer is:

algebraist

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

'maths' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'expert's lager tab is recalculated' is the wordplay.
'lager tab is' anagrams to 'ALGEBRAIST'.
However, I am not sure how this is indicated.

Can you help me to learn more?

I've seen this clue in The Independent.

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.