Person of exceptional intelligence was hard to indoctrinate (9)
I believe the answer is:
brainwash
'indoctrinate' is the definition.
(brainwashing is a kind of indoctrinating)
'person of exceptional intelligence was hard' is the wordplay.
'person of exceptional intelligence' becomes 'brain' (a brainy person).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'brain'+'was'+'h'='BRAINWASH'
'to' is the link.
(Other definitions for brainwash that I've seen before include "Coerce into belief" , "Completely indoctrinate" , "Condition" , "Subject someone to stressful indoctrination" , "Apply pressure" .)