Old time theatre one enters (5,3)
I believe the answer is:
stone age
'old time' is the definition.
(I've seen this before)
'theatre one enters' is the wordplay.
'theatre' becomes 'stage' (stage is a kind of theatre).
'enters' is an insertion indicator.
'stage' placed around 'one' is 'STONE-AGE'.
(Other definitions for stone age that I've seen before include "Early period" , "25 [ERA]" , "Goat seen (anag.)" , "Hard time" , "Old time" .)