A sail indeed overwhelmed by the waves? (7)
I believe the answer is:
deluged
'overwhelmed by the waves?' is the definition.
'a sail indeed' is the wordplay.
I cannot really see how this works, but
'sail' could be 'lug' (lug is a kind of sail) and 'lug' is located in the answer.
The remaining letters 'deed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for deluged that I've seen before include "Flooded" , "Inundated" .)