Doubt one working after pursuit (8)
I believe the answer is:
Here is my best explanation:
'doubt' is the definition.
('question' can be a synonym of 'doubt')
'one working after pursuit' is the wordplay.
'one' becomes 'i' (Roman numeral).
'working' becomes 'on'.
'after' means one lot of letters go next to another.
'pursuit' becomes 'quest' ('quest' can be a synonym of 'pursuit').
'ion' after 'quest' is 'QUESTION'.