Regret backing areas, including South Island, as large land mass (7)
I believe the answer is:
eurasia
'large land mass' is the definition.
(landmass including Europe and Asia)
'regret backing areas including south island' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'backing' is a reversal indicator.
'areas' becomes 'AA' (two abbreviations for area).
'including' means one lot of letters goes inside another.
'south island' becomes 'si' (this could be a standard abbreviation I've not previously seen).
'rue' backwards is 'eur'.
'aa' placed around 'si' is 'asia'.
'eur'+'asia'='EURASIA'
'as' is the link.
(Other definitions for eurasia that I've seen before include "Land mass - use aria (anag)" , "Large northern hemisphere land mass" , "Largest landmass" , "Large landmass combining two continents" , "Germany and Japan are in it" .)