Pointless empty unit in document holder (6)
I believe the answer is:
futile
'pointless' is the definition.
(I've seen this in another clue)
'empty unit in document holder' is the wordplay.
'empty' means to remove the middle letters.
'in' is an insertion indicator.
'document holder' becomes 'file' (a file might contain documents).
'unit' with its centre taken out is 'ut'.
'ut' put inside 'file' is 'FUTILE'.
(Other definitions for futile that I've seen before include "Useless, a waste of effort" , "Ineffectual, vain" , "Unavailing, a waste of time" , "Ineffectual,in vain" , "In vain, a waste of time" .)