When fire was harnessed (5,3)
I believe the answer is:
stone age
'when fire was harnessed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stone age that I've seen before include "Early period" , "before copper came in" , "Neolithic" , "Goat seen (anag.)" , "Go, senate to ancient era before metal was discovered" .)