Once a stormy sea (5)
I believe the answer is:
ocean
'sea' is the definition.
(synonyms)
'once a stormy' is the wordplay.
'stormy' indicates anagramming the letters (stormy suggests disruption).
'once'+'a'='oncea'
'oncea' with letters rearranged gives '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" .)