Stayed in custody (8)
I believe the answer is:
arrested
'in custody' is the definition.
(I've seen this in another clue)
'stayed' is the wordplay.
I cannot really understand how this works, but
'stayed' could be 'rested' ('rest' can be a synonym of 'stay') and 'rested' is located in the answer.
The remaining letters 'ar' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for arrested that I've seen before include "on the way to . . . [PRISON] ?" , "Taken into police custody" , "Seized and took into custody" , "Apprehended by police" , "Apprehended and taken into custody" .)