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 subsidiary indication.
'one' becomes 'i' (roman numeral).
'working' becomes 'on'.
'after' says to put letters next to each other.
'pursuit' becomes 'quest' ('quest' can be a synonym of 'pursuit').
'ion' put after 'quest' is 'question'.
(Other definitions for question that I've seen before include "Pump", ""To be or not to be", for example", "See 17", "dispute", "Challenge", "What could initiate this", "Call into doubt", "motion for debate", "Poser" and "Enquiry; doubt".)