Great intelligence one found in part of family (6)
I believe the answer is:
genius
'great intelligence' is the definition.
(I've seen this before)
'one found in part of family' is the wordplay.
'one' becomes 'I' (Roman numeral).
'found in' indicates putting letters inside.
'part of family' becomes 'genus' (biological taxonomy).
'i' inserted into 'genus' is 'GENIUS'.
(Other definitions for genius that I've seen before include "One has an extraordinary talent" , "egghead" , "Virtuoso" , "One has outstanding creative or intellectual ability" , "Exceptional intelligence" .)