Some professor, a clever expert (6)
I believe the answer is:
oracle
'expert' is the definition.
(I've seen this before)
'some professor a clever' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ORACLE' can be found hidden inside 'professor a clever'.
(Other definitions for oracle that I've seen before include "Authoritative person who divines the future" , "O, clear off for the Greek wise man" , "Ancient Greek expounder of wisdom" , "Shrine; its message" , "One believed to give divinely-inspired answers" .)