Humorless (4)
I believe the answer is:
grim
'humorless' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for grim that I've seen before include "Serious, forbidding" , "looking black" , "(Of humour) black; unappealing, depressing" , "Serious; dour" , "The - Reaper is death" .)