Simplistic account contained in dossier (6)
I believe the answer is:
facile
'simplistic' is the definition.
(I've seen this before)
'account contained in dossier' is the wordplay.
'account' becomes 'ac' (abbreviation for account).
'contained in' means one lot of letters goes inside another.
'dossier' becomes 'file' (a dossier is a file or set of information).
'ac' placed into 'file' is 'FACILE'.
(Other definitions for facile that I've seen before include "Easily achieved but of little value" , "Smooth" , "Over-simple, superficial" , "Arrived at without due care or effort" , "Lacking depth, argument say" .)