Marine mollusc that damages ships (6)
I believe the answer is:
teredo
'marine mollusc that damages ships' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for teredo that I've seen before include "Boring mollusc" , "Something getting into wood" , "Shipworm - red toe (anag)" , "Marine mollusc that bores into ships" , "Marine mollusc that damages ships" .)