Humourless, solemn (7)
(Other definitions for po-faced that I've seen before include "Wearing a stern expression" , "Glum" , "Informally solemn in appearance, like nocturnal vessel?" , "Not showing amusement, having solemn expression" , "Humourless and disapproving" .)
(Another definition for pofaced that I've seen is " visibly disapproving".)