A patch of sea, really? (4)
I believe the answer is:
area
'a patch' is the definition.
(I've seen this before)
'of sea really?' is the wordplay.
'of' says the answer is hidden in the clue.
'AREA' is hidden within 'sea really'.
(Other definitions for area that I've seen before include "part of town" , "Space for a specific purpose, say picnic" , "Length multiplied by breadth" , "land" , "Geographical region" .)