Chamber got used to being solemn (2-5)
I believe the answer is:
po-faced
'solemn' is the definition.
Both the definition and answer are adjectives.
Maybe they are linked in a way I don't understand?
'chamber got used to being' is the wordplay.
I cannot really see how this works, but
'chamber' could be 'po' (I've seen this in other clues) and 'po' is found within the answer.
'being' could be 'face' (face is a kind of being) and 'face' is present in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for po-faced that I've seen before include "Glum" , "With a disapproving countenance" , "Humourlessly disapproving" , "Looking unamused" , "Humourless and disapproving" .)