Mark exam questions, written on this? (9)
I believe the answer is:
notepaper
'questions written on this?' is the definition.
(you might write questions on notepaper)
'mark exam' is the wordplay.
'mark' becomes 'note' ('note' can be a synonym of 'mark').
'exam' becomes 'paper'.
'note'+'paper'='NOTEPAPER'
(Other definitions for notepaper that I've seen before include "What correspondent needs" , "Sheets for memos" , "perhaps, a contributor to writer's block" , "Letters are sent on it" , "What letter writer uses" .)