Edna goes back to southern mountains (5)
I believe the answer is:
andes
'mountains' is the definition.
(I've seen this before)
'edna goes back to southern' is the wordplay.
'goes back' says the letters should be written backwards.
'to southern' becomes 's' (abbreviation).
'edna' written backwards gives 'ande'.
'ande'+'s'='ANDES'
(Other definitions for andes that I've seen before include "Aconcagua the highest in this range" , "South American mountain chain" , "Mountain range in South America" , "South American mountain system" , "Great mountains of western S America" .)