It can be used as a bulwark against flood (7)
I believe the answer is:
sandbag
'it can be used as a bulwark against flood' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sandbag that I've seen before include "Attack" , "defensive article" , "One piled for flood protection" , "Item used for flood protection" , "It's used as a bulwark against flood" .)