A lot of water in room upstairs - get daughter out (8)
I believe the answer is:
atlantic
'a lot of water' is the definition.
'in room upstairs get daughter out' is the wordplay.
'in' means one lot of letters goes inside another.
'room upstairs' becomes 'attic'.
'get' becomes 'land' (landing is a kind of getting).
'daughter' becomes 'd' (genealogical abbreviation).
'out' indicates named letters should be taken away.
'land' with 'd' taken out is 'lan'.
'attic' going around 'lan' is 'ATLANTIC'.
(Other definitions for atlantic that I've seen before include "'The Pond'" , "Latin cat strangely in the sea" , "main" , "Azores, Falklands, in it" , "Western ocean." .)