Forbidding rear (5)
I believe the answer is:
stern
I believe this is a double definition.
'forbidding' is the first definition.
(synonyms)
'rear' is the second definition.
(I know that rear can be written as stern)
(Other definitions for stern that I've seen before include "Unyielding part of the ship" , "Strict; severe" , "Unpleasantly serious" , "Authoritarian, back" , "Old star" .)