Impertinent fool stretches the bounds of stupidity (5)
I believe the answer is:
sassy
'impertinent' is the definition.
(thesaurus)
'fool stretches the bounds of stupidity' is the wordplay.
'fool stretches' becomes 'sas' (I can't justify this - if you can you should believe this answer much more).
'the bounds of' says to hollow out the word (remove centre letters) (the letters which form the boundaries of the word).
'stupidity' with its centre removed is 'sy'.
'sas'+'sy'='SASSY'
Can you help me to learn more?
(Other definitions for sassy that I've seen before include "Bold, spirited, cheeky (Amer.)" , "Cheeky (US colloq.)" , "Pert" , "(In US) saucy" , "Cheeky American" .)