Stage includes one ancient era (5,3)
I believe the answer is:
stone age
'ancient era' is the definition.
'Stone Age' can be an answer for 'era' (thesaurus). I am unsure of the 'ancient' bit.
'stage includes one' is the wordplay.
'includes' is an insertion indicator.
'stage' enclosing 'one' is 'STONE-AGE'.
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "25 [ERA]" , "Earliest known period of human culture" , "Pre-historic era" , "Early epoch when man used pre-metal tools" , "Old time" .)