Individual entered theatre a long time ago (5,3)
I believe the answer is:
stone age
'a long time ago' is the definition.
(I've seen this before)
'individual entered theatre' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'entered' is an insertion indicator.
'theatre' becomes 'stage' (stage is a kind of theatre).
'one' placed within 'stage' is 'STONE-AGE'.
(Other definitions for stone age that I've seen before include "Earliest known period of human culture" , "Go, senate to ancient era before metal was discovered" , "time long ago" , "Goat seen (anag.)" , "Early period" .)