Liquid starter from waitress, so doubled up, put in sketch? (3,5)

'liquid starter from waitress' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'so doubled up put in sketch?' is the wordplay.
I cannot quite understand how this works, but
'so' is within the answer.
'up' is present in the answer.
The remaining letters 'twos' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?