In the year before year one: nothing! - extremely frustrating (8)
I believe the answer is:
annoying
'frustrating' is the definition.
'in the year before year one nothing extremely' is the wordplay.
'in the year' becomes 'anno' (I've seen this before).
'before' means one lot of letters go next to another.
'year' becomes 'y'.
'one' becomes 'i' (Roman numeral).
'extremely' says to hollow out the word (remove centre letters).
'nothing' with its centre taken out is 'ng'.
'anno'+'y'+'i'+'ng'='ANNOYING'
(Other definitions for annoying that I've seen before include "Irksome" , "Vexing" , "Irritating, bothersome" , "Vexatious" , "Disturbing, irritating" .)