Titanic's nemesis (7)
I believe the answer is:
iceberg
'titanic's nemesis' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iceberg that I've seen before include "Main hazard" , "Type of floe" , "Large floating mass of frozen water" , "Floating mass (of lettuce?)" , "What sunk the Titanic" .)