The last letter before each large number for a fanatic (6)
I believe the answer is:
zealot
'for a fanatic' is the definition.
(both can mean an extremist)
'the last letter before each large number' is the wordplay.
'the last letter' becomes 'Z' (last letter of the alphabet).
'before' is a charade indicator (letters next to each other).
'each' becomes 'ea' (abbreviation).
'large number' becomes 'lot' (a large amount).
'z'+'ea'+'lot'='ZEALOT'
(Other definitions for zealot that I've seen before include "Radical" , "A fanatic or over-enthusiastic person" , "Fervent, even militant, proponent of something" , "Fervent partisan" , "Fanatic, fiery follower" .)