Impertinence, being still upset about excellence (8)
I believe the answer is:
temerity
'impertinence' is the definition.
(thesaurus)
'still upset about excellence' is the wordplay.
'still' becomes 'yet' (yet can mean still or so far).
'upset' shows that the letters should be reversed in order.
'about' is an insertion indicator.
'excellence' becomes 'merit' (I've seen this before).
'yet' back-to-front is 'tey'.
'tey' enclosing 'merit' is 'TEMERITY'.
'being' is the link.
(Other definitions for temerity that I've seen before include "Excessive boldness" , "Gall" , "Rashness, audacity" , "Brashness or rashness" , "Overconfidence" .)