Area used to be hot and covered in water (5)
I believe the answer is:
awash
'covered in water' is the definition.
(I've seen this before)
'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'
'and' acts as a link.
(Other definitions for awash that I've seen before include "Flooded; about to sink" , "becomes flooded" , "Deluged" , "Covered by water - sounds cleansing" , "Drenched" .)