Open (a fastener) (5)
I believe the answer is:
unzip
'open a fastener' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unzip that I've seen before include "Unfasten" , "Loosen fastening" , "Loosen (trousers?)" , "Undo trousers" , "Release covering (or trousers?)" .)