Clever like two thirds of private pupils (6)
I believe the answer is:
astute
'clever' is the definition.
(astute means sharp or clever)
'like two thirds of private pupils' is the wordplay.
I cannot really understand how this works, but
'like' could be 'as' and 'as' is found within the answer.
The remaining letters 'tute' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for astute that I've seen before include "Clever and perceptive" , "Shrewd, quick to spot advantage" , "Able to make accurate assessments" , "Clever and crafty" , "Shrewd and sagacious" .)