In compartment stand opposite, looking humourless (2-5)
I believe the answer is:
po-faced
'looking humourless' is the definition.
(po-faced means overly prudish or disapproving)
'in compartment stand opposite' is the wordplay.
'in' is an insertion indicator.
'compartment' becomes 'pod' (pod can mean a compartment or case).
'stand opposite' becomes 'face' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pod' going around 'face' is 'PO-FACED'.
Can you help me to learn more?
(Other definitions for po-faced that I've seen before include "Looking unamused" , "Wearing a stern expression" , "Humourlessly disapproving" , "With a disapproving countenance" , "Not showing amusement, having solemn expression" .)