Coming from the sea, unexpectedly (3,2,3,4)
I believe the answer is:
out of the blue
'unexpectedly' is the definition.
('out of the blue' can be a synonym of 'unexpectedly')
'coming from the sea' is the wordplay.
'coming from' becomes 'outof' (out of).
'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 "Entirely unexpectedly" , "Without warning" , "Quite unexpectedly" , "Suddenly" , "From nowhere" .)