Make waterproof (4)
I believe the answer is:
seal
'make waterproof' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seal that I've seen before include "Close - (performing?) animal" , "Bull or cow" , "Aquatic mammal - close securely" , "Marine animal or sign of authenticity" , "Amphibious mammal or stamp of authenticity" .)