Peninsula in Russian region sons must leave (6)
I believe the answer is:
iberia
'peninsula' is the definition.
(peninsula including Spain and Portugal)
'russian region sons must leave' is the wordplay.
'russian region' becomes 'Siberia' (Siberia is part of Russia).
'sons' becomes 's'.
'must leave' is a deletion indicator.
'siberia' with 's' taken away is 'IBERIA'.
'in' acts as a link.
(Other definitions for iberia that I've seen before include "European territory" , "Peninsula encompasing Spain and Protugal" , "old part of Europe" , "Old name for Spanish peninsula" , "Peninsula that was Roman province" .)