Once a stormy sea (5)
I believe the answer is:
ocean
'sea' is the definition.
(I know that sea can be written as ocean)
'once a stormy' is the wordplay.
'stormy' is an anagram indicator (stormy suggests disruption).
'once'+'a'='oncea'
'oncea' is an anagram of 'OCEAN'.
(Other definitions for ocean that I've seen before include "Big sea" , "Indian, for example" , "tons of water" , "Indian or Arctic, say" , "Large body of salt water" .)