Detain (6)
'detain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arrest that I've seen before include "Stop (on suspicion)" , "Take into custody as police do" , "Stop, check (progress)" , "Bring to a standstill" , "apprehend a law-breaker" .)
(Other definitions for intern that I've seen before include "Hold in custody" , "Often unpaid trainee" , "Student" , "Imprison (enemy aliens)" , "Confine - assistant medic in America" .)