One completes one's task with conviction (10)
I believe the answer is:
prosecutor
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for prosecutor that I've seen before include "originator of suit" , "Court official" , "One making case against defendant" , "One stating the case against a defendant" , "One who conducts case against defendant" .)