Unexpectedly from the sea (3,2,3,4)
I believe the answer is:
out of the blue
'unexpectedly' is the definition.
(synonyms)
'from the sea' is the wordplay.
'from' becomes 'out of' (I've seen this in another clue).
'the sea' becomes 'theblue' (the blue).
'outof'+'theblue'='OUT OF THE BLUE'
(Other definitions for out of the blue that I've seen before include "Without warning" , "Entirely unexpectedly" , "Quite unexpectedly" , "From nowhere" , "Suddenly" .)