Cuttlefish defense (3)
I believe the answer is:
ink
'cuttlefish defense' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ink that I've seen before include "It is ejected by cuttlefish" , "Liquid used in printing" , "Squid's means of concealment" , "Cuttlefish's secretion" , "Cuttlefish defence" .)