Ship's cable (6)
I believe the answer is:
hawser
'ship's cable' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hawser that I've seen before include "A heavy rope or cable for mooring ship" , "Heavy rope used at sea" , "Ship's tow-rope" , "Heavy rope for nautical use" , "Quayside rope" .)