Very old style adopted by philosopher (5-3)
I believe the answer is:
stone age
'very old' is the definition.
(I've seen this before)
'style adopted by philosopher' is the wordplay.
'style' becomes 'tone' (tone is a kind of style).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'philosopher' becomes 'sage' (sage can mean a wise person).
'tone' placed inside 'sage' is 'STONE-AGE'.
(Other definitions for stone age that I've seen before include "Pre-historic era" , "Old time" , "Early epoch when man used pre-metal tools" , "Early period" , "Prehistoric period" .)