Allow no openings for substitutes (8)
I believe the answer is:
stopgaps
'for substitutes' is the definition.
(I have seen 'Temporary substitutes ' mean 'stopgaps' so perhaps 'substitutes' could also mean 'stopgaps')
'allow no openings' 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).
'openings' becomes 'gaps' ('gap' can be a synonym of 'opening').
'stop'+'gaps'='STOPGAPS'
Can you help me to learn more?
(Other definitions for stopgaps that I've seen before include "Temporary measures" , "Temporary substitutes to plug openings?" , "Temporary substitutes, like fingers in dykes" , "Temporary substitutes or fixes" , "Temporary substitutes or expedients" .)