Part of a schoolboy essay that's correct (3)
I believe the answer is:
yes
'that's correct' is the definition.
(I've seen this in another clue)
'part of a schoolboy essay' is the wordplay.
'part of' says the answer is hidden in the clue.
'YES' is hidden within 'schoolboy essay'.
(Other definitions for yes that I've seen before include "Validating vote" , "Word of affirmation" , "Certainly!" , "One response to "will you marry me?"" , "Pro vote" .)