A clout will often secure an admission (5)
I believe the answer is:
knock
'a clout' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for knock that I've seen before include "Fail to hit" , "Bang" , "Tap on door" , "Strike; setback" , "Setback; thirteen across" .)