Performing lead in Equus in theatre for a period (5,3)
I believe the answer is:
stone age
'a period' is the definition.
(Stone Age is an example)
'performing lead in equus in theatre' is the wordplay.
'performing' becomes 'on' (I've seen this in other clues).
'lead in' says to take the initial letters.
'in' is an insertion indicator.
'theatre' becomes 'stage' (stage is a kind of theatre**).
The first letter of 'equus' is 'e'.
'on'+'e'='one'
'one' going inside 'stage' is 'STONE-AGE'.
'for' is the link.
(Other definitions for stone age that I've seen before include "before copper came in" , "Goat seen (anag.)" , "Pre-historic era" , "Primitive period" , "Early epoch when man used pre-metal tools" .)