Sea creature protects a bit of land (4)
I believe the answer is:
acre
'a bit of land' is the definition.
(unit of land area)
'sea creature protects' is the wordplay.
'protects' says the answer is hidden in the clue (hidden letters are protected by the clue).
'ACRE' is hidden within 'sea creature'.
(Other definitions for acre that I've seen before include "amount of land" , "Site of battle" , "Four roods - Israeli port" , "Care about the piece of land" , "Unit of area - Israeli port" .)