The naughtiness of showing insufficient zeal at a jumble sale (6)
I believe the answer is:
sleaze
'the naughtiness of' is the definition.
The definition suggests a singular noun which matches the answer.
'insufficient zeal at a jumble sale' is the wordplay.
'insufficient zeal' becomes 'ze' (I can't justify this - if you can you should give a lot more credence to this answer).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'a jumble' indicates anagramming the letters.
'sale' with letters rearranged gives 'slea'.
'ze' after 'slea' is 'SLEAZE'.
'showing' is the link.
Can you help me to learn more?
(Other definitions for sleaze that I've seen before include "Sordidness, immorality" , "Vulgarity" , "Sordid political behaviour?" , "Corruption in public life (informal)" , "'Cheapness, tackiness (6)'" .)