Enter into an arrangement for custody (9)
I believe the answer is:
retention
'custody' is the definition.
(I've seen this before)
'enter into an arrangement' is the wordplay.
'an arrangement' indicates an anagram.
'enter'+'into'='enterinto'
'enterinto' anagrammed gives 'RETENTION'.
'for' is the link.
(Other definitions for retention that I've seen before include "Capacity to remember" , "Failure to eliminate a substance from the body" , "maintenance" , "Saving" , "Keeping hold (of something)" .)