Flooded area used to be hot (5)
I believe the answer is:
awash
'flooded' is the definition.
('awash' can be a synonym of 'flooded')
'area used to be hot' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'used to be' becomes 'was' (I've seen this before).
'hot' becomes 'h'.
'a'+'was'+'h'='AWASH'
(Other definitions for awash that I've seen before include "Letting in the sea" , "Level with the water surface" , "Covered by water - sounds cleansing" , "Swimming" , "not quite under" .)