Prevent opening being makeshift (7)
I believe the answer is:
stopgap
'makeshift' is the definition.
(synonyms)
'prevent opening' is the wordplay.
'prevent' becomes 'stop' (stopping is a kind of preventing).
'opening' becomes 'gap' (gap is a kind of opening).
'stop'+'gap'='STOPGAP'
'being' is the link.
(Other definitions for stopgap that I've seen before include "A temporary substitute or solution" , "Makeshift" , "Temporary fix" , "caretaker" , "Temporary measure" .)