Crossword Genius

In a restaurant, a helpful line in cheap drink? (6)

Ross

I believe the answer is:

waiter

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

'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'a restaurant a helpful line in cheap drink?' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
'drink?' could be 'it' and 'it' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for waiter that I've seen before include "Table server" , "One paid to serve" , "Estragon or Vladimir, for example" , "One who serves at table" , "Servant in bar" .)

I've seen this clue in the Evening Standard.
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