From a bygone age, nasty one fired bumpkin slightly lacking (9)
I believe the answer is:
neolithic
'from a bygone age' is the definition.
(from the Neolithic period)
'nasty one fired bumpkin slightly lacking' is the wordplay.
'nasty' indicates an anagram (a nasty or wrong spelling).
'fired' becomes 'lit' (set on fire).
'bumpkin' becomes 'hick' (synonyms).
'slightly lacking' means to remove the last letter.
'hick' with its last letter taken away is 'hic'.
'one' with letters rearranged gives 'neo'.
'neo'+'lit'+'hic'='NEOLITHIC'
(Other definitions for neolithic that I've seen before include "about when stone weapons prevailed" , "Belonging to late Stone Age" , "Description of ancient people" , "Of New Stone Age" , "Description of old age" .)