Green Gables girl in Johannesburg (4)
I believe the answer is:
anne
'green gables girl' is the definition.
(as in the novel Anne of Green Gables)
'in johannesburg' is the wordplay.
'in' indicates a hidden word.
'ANNE' can be found hidden inside 'johannesburg'.
(Other definitions for anne that I've seen before include "Queen 1702-1714" , "Name of queen or princess" , "The Bard wooed and won her" , "One of two Henry VIII wives" , "Henry's second" .)