Teacher: Hannibal surrounds ancient city (8)
I believe the answer is:
lecturer
'teacher' is the definition.
(thesaurus)
'hannibal surrounds ancient city' is the wordplay.
'hannibal' becomes 'lecter'.
'surrounds' means one lot of letters goes inside another.
'ancient city' becomes 'ur'.
'lecter' enclosing 'ur' is 'LECTURER'.
(Other definitions for lecturer that I've seen before include "with a penchant for haranguing?" , "University teacher" , "Speaker" , "don provided" , "L" .)