Held in custody below, a leader of ring and others (5,6)
I believe the answer is:
under arrest
'held in custody' is the definition.
'below a leader of ring and others' is the wordplay.
'below' becomes 'under' ('under' can be a synonym of 'below'**).
'leader of' says to take the initial letters.
'and' is a charade indicator (letters next to each other).
'others' becomes 'rest' (I've seen this before).
The initial letter of 'ring' is 'r'.
'under'+'a'+'r'+'rest'='UNDER ARREST'
(Other definitions for under arrest that I've seen before include "help the police with their enquiries?" , "Detained" .)