Grim ship's tail end (5)
I believe the answer is:
stern
This clue is a quadruple definition!!
'grim' is the first definition.
(synonyms)
'ship's' is the second definition.
(thesaurus)
'tail' is the third definition.
(back of a ship or boat)
'end' is the fourth definition.
(I have seen 'Rear end ' mean 'stern' so perhaps 'end' could also mean 'stern')
(Other definitions for stern that I've seen before include "Severe; grimly serious" , "Dour" , "Harsh at the back of the ship" , "Looking severe" , "Severe in the rear of the ship?" .)