Surprisingly enter into custody (9)
I believe the answer is:
retention
'custody' is the definition.
(I've seen this before)
'surprisingly enter into' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'enter'+'into'='enterinto'
'enterinto' is an anagram of 'RETENTION'.
(Other definitions for retention that I've seen before include "Enter into custody" , "Failure to eliminate a substance from the body" , "maintenance" , "Keeping hold (of something)" , "memory" .)