Naughty nun's feat is to unzip! (8)
I believe the answer is:
unfasten
'to unzip' is the definition.
'naughty nun's feat' is the wordplay.
'naughty' indicates an anagram.
'nuns'+'feat'='nunsfeat'
'nunsfeat' is an anagram of 'UNFASTEN'.
'is' is the link.
(Other definitions for unfasten that I've seen before include "Loosen or open, say seatbelt" , "Let go" , "Nun's fate (anag.)" , "Unlock" , "Undo" .)