Open feast nun prepared (8)
I believe the answer is:
unfasten
'open' is the definition.
(unfastening is a kind of opening)
'feast nun prepared' is the wordplay.
'prepared' is an anagram indicator.
'feast'+'nun'='feastnun'
'feastnun' with letters rearranged gives 'UNFASTEN'.
(Other definitions for unfasten that I've seen before include "Nun's fate (anag.)" , "Let go" , "tie? No" , "Loosen or open, say seatbelt" , "Unlock" .)