Allow no opening for a substitute (7)
I believe the answer is:
stopgap
'for a substitute' is the definition.
(I've seen this before)
'allow no opening' is the wordplay.
'allow no' becomes 'stop' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'opening' becomes 'gap' (synonyms).
'stop'+'gap'='STOPGAP'
Can you help me to learn more?
(Other definitions for stopgap that I've seen before include "A temporary or impromptu substitute" , "Interim" , "Temporary measure" , "Makeshift" , "Temporary fix" .)