Approach taken by wise person in olden days (5,3)
I believe the answer is:
stone age
'approach taken by wise' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'person in olden days' is the wordplay.
'person' becomes 'one' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'olden days' becomes 'stage' (I can't justify this - if you can you should give a lot more credence to this answer).
'one' put inside 'stage' is 'STONE-AGE'.
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "Old time" , "Prehistoric period" , "Go, senate to ancient era before metal was discovered" , "before copper came in" , "Early period" .)