Main area once a disaster (5)
I believe the answer is:
ocean
'main' is the definition.
(synonyms)
'area once a disaster' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'a disaster' indicates anagramming the letters.
'a'+'once'='aonce'
'aonce' is an anagram of 'OCEAN'.
(Other definitions for ocean that I've seen before include "Large stretch of water" , "Great sea" , "Vast quantity of liquid" , "Immense expanse or quantity" , "Large amount of water and" .)