Mostly benevolent western sultanate imprisoning second family member (9)
I believe the answer is:
kinswoman
'family member' is the definition.
'mostly benevolent western sultanate imprisoning second' is the wordplay.
'mostly benevolent' becomes 'kin' (I can't justify this - if you can you should believe this answer much more).
'western' becomes 'w'.
'sultanate' becomes 'oman' (I've seen this before).
'imprisoning' indicates putting letters inside.
'second' becomes 's' (common abbreviation).
'kin'+'w'+'oman'='kinwoman'
'kinwoman' enclosing 's' is 'KINSWOMAN'.
Can you help me to learn more?
(Other definitions for kinswoman that I've seen before include "Relation" , "Female family member" , "Female relative" , "one of the family!" .)