Detain worker — for nothing? (6)
I believe the answer is:
intern
I believe this clue is a double definition.
'detain' is the first definition.
(I know that intern is a more specific form of the action detain)
'worker for nothing?' is the second definition.
(interns often work for free)
(Other definitions for intern that I've seen before include "Medical graduate trainee" , "Imprison the young doctor without trial" , "Imprison (eg, alien) as a precautionary measure" , "House officer" , "Often unpaid trainee" .)