Do put in dossier as ‘pointless’ (6)
I believe the answer is:
futile
'pointless' is the definition.
(thesaurus)
'do put in dossier' is the wordplay.
'do' becomes 'ut' ('ut' can be a synonym of 'do').
'put in' means one lot of letters goes inside another.
'dossier' becomes 'file' (I've seen this before).
'ut' put inside 'file' is 'FUTILE'.
'as' acts as a link.
(Other definitions for futile that I've seen before include "Ineffectual, a waste of time" , "Effectless" , "Trifling and ineffectual" , "Unavailing, a waste of time" , "Ineffectual,in vain" .)