Hold up – keep in custody (6)
I believe the answer is:
detain
I believe this clue is a double definition.
'hold up' is the first definition.
(I know that hold up can be written as detain)
'keep in custody' is the second definition.
(I've seen this before)
(Other definitions for detain that I've seen before include "Hold back - arrest" , "Keep in custody; hold back" , "Confine" , "Take into confinement" , "Hold back, perhaps for questioning" .)