Mostly gas-fired, this Latin period (9)
I believe the answer is:
neolithic
'period' is the definition.
(neolithic period)
'mostly gas-fired this latin' is the wordplay.
'mostly' means to remove the last letter.
'gas' becomes 'neon' (I've seen this before).
'fired' becomes 'lit' (set on fire).
'this latin' becomes 'hic' ('this' in Latin).
'neon' with its last letter taken away is 'neo'.
'neo'+'lit'+'hic'='NEOLITHIC'
(Other definitions for neolithic that I've seen before include "Belonging to late Stone Age" , "Of the later part of the Stone Age" , "Stone-Age" , "Description of old age" , "about when stone weapons prevailed" .)