Doubt one working after pursuit (8)
I believe the answer is:
question
'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').
'i'+'on'='ion'
'ion' after 'quest' is 'QUESTION'.
(Other definitions for question that I've seen before include "Pump" , "Call into doubt" , "motion for debate" , "dispute" , "Query" .)