A stiff back? (5)
I believe the answer is:
stern
I believe this is a double definition.
'a stiff' is the first definition.
(thesaurus)
'back?' is the second definition.
(stern is a kind of back)
(Other definitions for stern that I've seen before include "Back end of a ship" , "Grim, strict" , "Severe; grimly serious" , "Rear end" , "Hard - back" .)