First of all, American western films upset Leone -- now even Sergio shoots horror (9)
I believe the answer is:
awfulness
'even sergio shoots horror' is the definition.
I can't tell whether this defines the answer.
'first of all american western films upset leone now' is the wordplay.
'first of all' says to take the initial letters.
'now' becomes 'ness' (I am not sure about this - if you are sure you should believe this answer much more).
The first letters of 'american western films upset leone' is 'awful'.
'awful'+'ness'='AWFULNESS'
Can you help me to learn more?
(Another definition for awfulness that I've seen is " Quality of being horrid".)