Not fastened (6)
I believe the answer is:
undone
'not fastened' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for undone that I've seen before include "Ruined - no longer fastened" , "Not finished; untied" , "Ruined or dismantled" , "Annulled; untied" , "Unfastened or brought to ruin" .)