Humourless fellow meets expert in school (2-5)
I believe the answer is:
po-faced
'humourless' is the definition.
(I've seen this before)
'fellow meets expert in school' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'meets' means one lot of letters go next to another.
'expert' becomes 'ace' (ace is a kind of expert).
'in' is an insertion indicator.
'school' becomes 'pod' (I've seen this before).
'f'+'ace'='face'
'face' inserted inside 'pod' is 'PO-FACED'.
(Other definitions for po-faced that I've seen before include "Humourlessly disapproving" , "With disapproving look" , "Humourless and disapproving" , "With a disapproving countenance" , "Glum" .)