Once a tempestuous expanse of water (5)
I believe the answer is:
ocean
'expanse of water' is the definition.
(I've seen this before)
'once a tempestuous' is the wordplay.
'tempestuous' indicates anagramming the letters.
'once'+'a'='oncea'
'oncea' anagrammed gives 'OCEAN'.
(Other definitions for ocean that I've seen before include "Huge expanse" , "A mighty sea" , "Pacific, perhaps" , "Vast quantity of liquid" , "One of the Seven Seas" .)