Popular girl to west of Ohio? (7)
I believe the answer is:
indiana
'to west of ohio?' is the definition.
(US state to the west of Ohio)
'popular girl' is the wordplay.
'popular' becomes 'in'.
'girl' becomes 'diana' (**).
'in'+'diana'='INDIANA'
(Other definitions for indiana that I've seen before include "Midwestern state" , "State of southern US" , "The Hoosier State" , "___ Jones, Harrison Ford hero - US state" , "--- Jones, Harrison Ford film character" .)