parergon
A piece of work that is supplementary to or a byproduct of a larger work.
[…] the running and control of double agents is a[n] […] infinitely complicated task. It could not be done as a parergon by any intelligence department elsewhere […]
Where it comes from
From Ancient Greek πάρεργον (párergon, “subordinate or secondary business; by-work”), neuter of πάρεργος (párergos, “beside the main subject, subordinate, incidental”).
Adapted from the Wiktionary entry parergon (CC BY-SA 4.0).