A custom among unscrupulous agents (5)
I believe the answer is:
usage
'a custom' is the definition.
(I know that custom can be written as usage)
'among unscrupulous agents' is the wordplay.
'among' says the answer is hidden in the clue.
'USAGE' can be found hidden inside 'unscrupulous agents'.
(Other definitions for usage that I've seen before include "Terminology - custom" , "Treatment, practice" , "Accepted or habitual practice" , "Tradition - rate of consumption" , "management" .)