Rigid back (5)
I believe the answer is:
stern
I believe this clue is a double definition.
'rigid' 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 "Demanding < back" , "Dour" , "Unrelenting at the back of the boat?" , "Inflexible; back" , "Authoritarian, back" .)