Upstart bitten by a cur so incredibly harshly loud (7)
I believe the answer is:
raucous
'harshly loud' is the definition.
(I've seen this before)
'upstart bitten by a cur so incredibly' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'bitten by' means one lot of letters goes inside another (I've seen this in other clues).
'incredibly' indicates anagramming the letters.
'a'+'cur'+'so'='acurso'
'acurso' is an anagram of 'racous'.
'u' going within 'racous' is 'RAUCOUS'.
(Other definitions for raucous that I've seen before include "Harsh in sound" , "Shrill - strident" , "Loud and discordant" , "Harsh, noisy" , "Unpleasantly noisy" .)