United lifted in first leg a long time ago (5,3)
I believe the answer is:
stone age
'long time ago' is the definition.
(I've seen this before)
'united lifted in first leg a' is the wordplay.
I cannot really see how this works, but
'united' could be 'one' (as in a phrase like 'we are one') and 'one' is located in the answer.
'first' could be 'a' (first letter of the alphabet) and 'a' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "Old time" , "Early period" , "Neolithic" , "Pre-historic era" , "Early epoch when man used pre-metal tools" .)