Useless dossier -- just regularly missing inside (6)
I believe the answer is:
futile
'useless' is the definition.
(similar in meaning)
'dossier just regularly missing inside' is the wordplay.
'dossier' becomes 'file' (I've seen this before).
'regularly missing' indicates alternate letters (letters removed at regular intervals).
'inside' means one lot of letters goes inside another.
The alternate letters of 'just' are 'ut'.
'file' enclosing 'ut' is 'FUTILE'.
(Other definitions for futile that I've seen before include "Unavailing, a waste of time" , "If lute is altered it becomes ineffectual." , "Useless or vain" , "Ineffectual,in vain" , "Ineffective" .)