Noun replacing quiet feeble sort is potty (5)
I believe the answer is:
nutty
'is potty' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nutty that I've seen before include "Crazy - slack?" , "Mad - with small pieces of coal" , "passionate" , "Mentally unhinged (slang)" , "Like cobs, brazils, etc" .)