One is held in custody (8)
I believe the answer is:
detainee
'one is held in custody' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for detainee that I've seen before include "One held in custody" , "One is held under guard, for political reasons perhaps" , "Someone held in custody" , "Prisoner" , "Person kept in custody" .)