Southern peninsula, a massive area (7)
I believe the answer is:
siberia
'a massive area' is the definition.
'southern peninsula' is the wordplay.
'southern' becomes 's' (abbreviation).
'peninsula' becomes 'iberia' (Iberia is an example).
's'+'iberia'='SIBERIA'
(Other definitions for siberia that I've seen before include "Arctic region" , "Eastern region of Russia" , "Region of Russia, from the Urals to the Pacific, famously cold in winter" , "'Region of Russia, famous for its long, cold winters (7)'" , "Cold area" .)