Grimly serious (5)
I believe the answer is:
stern
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grimly serious</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for stern that I've seen before include "Forbidding in aspect" , "Authoritarian, back" , "End part" , "having a forbidding appearance" , "Rear; severe, serious" .)