On which tennis players may be all at sea! (4)
I believe the answer is:
deck
'sea' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for deck that I've seen before include "Horizontal floor on a ship" , "Adorn; level on ship" , "part of ship" , "Floor on board" , "fell to floor" .)