Block, with "off" (4)
I believe the answer is:
seal
'block with off' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for seal that I've seen before include "Sea-dweller can produce a wax impression" , "Settle or decide - say fate" , "A mammal; a fastening" , "Stamped wax; mammal" , "Close up - marine mammal" .)