Thinking Greek child left during a hike (9)
I believe the answer is:
aristotle
'thinking greek' is the definition.
(I've seen this before)
'child left during a hike' is the wordplay.
'child' becomes 'tot' (tot is a kind of child).
'left' becomes 'l' (common abbreviation**).
'during' means one lot of letters goes inside another.
'hike' becomes 'rise' ('rise' can be a synonym of 'hike').
'tot'+'l'='totl'
'a'+'rise'='arise'
'totl' put into 'arise' is 'ARISTOTLE'.
(Other definitions for aristotle that I've seen before include "Realist to be like great Greek philosopher" , "Thinking Greek" , "Greek philosopher, pupil of Plato, tutor of Alexander the Great" , "philosophy tutor" , "Greek philosopher (Poetics, Politics)" .)