Somber
'somber' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for grim that I've seen before include "Very serious/gloomy" , "Dour, forbidding" , "Serious; dour" , "Dour and severe" , "formidable" .)
(Other definitions for stern that I've seen before include "Grim, strict" , "Unsmiling" , "Grave; rear" , "Severe part of the ship?" , "Tail of a beagle" .)