Humourless (2-5)
I believe the answer is:
po-faced
'humourless' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for po-faced that I've seen before include "Wearing a stern expression" , "Not showing amusement, having solemn expression" , "Informally solemn in appearance, like nocturnal vessel?" , "With a disapproving countenance" , "With disapproving look" .)