A left-winger's to remain inside, taken into custody (8)
I believe the answer is:
arrested
'taken into custody' is the definition.
(I've seen this before)
'a left-winger's to remain inside' is the wordplay.
'left-winger' becomes 'red' (red is a kind of left-winger).
'to remain' becomes 'rest' ('rest' can be a synonym of 'remain').
'inside' means one lot of letters goes inside another.
'red' going around 'rest' is 'rrested'.
'a'+'rrested'='ARRESTED'
(Other definitions for arrested that I've seen before include "on the way to . . . [PRISON] ?" , "Taken into custody, being serrated" , "Checked" , "Taken into custody or stopped" , "Detained by legal authority" .)