Stop prior to opening (7)
I believe the answer is:
prevent
'stop' is the definition.
(I've seen this before)
'prior to opening' is the wordplay.
'prior to' becomes 'pre' (I've seen this in other clues).
'opening' becomes 'vent' (vent is a kind of opening).
'pre'+'vent'='PREVENT'
(Other definitions for prevent that I've seen before include "Obviate" , "Keep from arising" , "Avert, hinder" , "Keep from happening" , "Stop, hinder" .)