Thwart before opening (7)
I believe the answer is:
prevent
'thwart' is the definition.
(I know that thwart is a more specific form of prevent)
'before opening' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'opening' becomes 'vent' (a vent is an opening or outlet).
'pre'+'vent'='PREVENT'
(Other definitions for prevent that I've seen before include "Thwart" , "Avert, hinder" , "Stop, avert" , "Stop from happening" , "Stop (an action)" .)