Similar in every detail, papers a client misplaced (9)
I believe the answer is:
identical
'similar in every detail' is the definition.
'papers a client misplaced' is the wordplay.
'papers' becomes 'ID' (identity papers).
'misplaced' indicates an anagram (letters in the wrong places).
'a'+'client'='aclient'
'aclient' is an anagram of 'entical'.
'id'+'entical'='IDENTICAL'
(Other definitions for identical that I've seen before include "not distinguished" , "Equal" , "Matching" , "Impossible to tell apart" , "Not any different" .)