A large wet area, and that's flat! (4,3)

'that's flat' is the definition.
I know nothing about this answer so I can't judge whether this works.

'a large wet area' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is present in the answer.
'area' could be 'a' (maths abbreviation) and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'and' acts as a link.

Can you help me to learn more?