Come down a great deal to continent (8)
I believe the answer is:
landmass
'to continent' is the definition.
(I've seen this before)
'come down a great deal' is the wordplay.
'come down' becomes 'land' (to land is to come down after flying).
'a great deal' becomes 'mass' ('mass' can be a synonym of 'great deal').
'land'+'mass'='LANDMASS'
(Other definitions for landmass that I've seen before include "Eg, a continent" , "Huge area" , "Asia, say" .)