The sea can be so blue (2,1,3,3)
I believe the answer is:
at a low ebb
'blue' is the definition.
(both can mean sad)
'the sea can be' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'be' could be 'owe' (owing is a kind of being) and 'owe' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'so' acts as a link.
Can you help me to learn more?
(Other definitions for at a low ebb that I've seen before include "Lacking vigour" , "In a weak state" , "In a depressed or weakened state" , "In weakened state, like tide gone out" , "Dispirited" .)