Period Toynbee regularly covers? On the contrary, this is prehistoric (5,3)
I believe the answer is:
stone age
'this is prehistoric' is the definition.
(part of prehistory)
'period toynbee regularly covers? on the contrary' is the wordplay.
'period' becomes 'age' (age is a kind of period).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'covers?' says to put letters next to each other.
'on' is a charade indicator (letters next to each other).
'the contrary' becomes 'st' (I am not sure about this - if you are sure you should believe this answer much more).
The alternating letters of 'toynbee' are 'one'.
'age' after 'one' is 'oneage'.
'oneage' after 'st' is 'STONE-AGE'.
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "before copper came in" , "Early period" , "Early epoch when man used pre-metal tools" , "Neolithic" , "Earliest known period of human culture" .)