Grim at the back
I believe the answer is:
stern
This clue is a double definition.
'grim' is the first definition.
(synonyms)
'the back' is the second definition.
(stern is a kind of back)
'at' acts as a link.
(Other definitions for stern that I've seen before include "Strict - forbidding" , "Demanding < back" , "Severe, demanding" , "Severe, forbidding" , "Grim; severe" .)