Free space, as in a large body of water (7,3)
I believe the answer is:
caspian sea
'large body of water' is the definition.
(the Caspian Sea is technically a large lake)
'free space as in a' is the wordplay.
'free' is an anagram indicator.
'space'+'as'+'in'='spaceasin'
'spaceasin' anagrammed gives 'caspianse'.
'caspianse'+'a'='CASPIAN SEA'
(Other definitions for caspian sea that I've seen before include "World's largest inland body of water, fed by the Volga River" , "Lake bordered by both Russia and Iran" , "large expanse of water" , "World's largest lake" , "Largest salt lake in the world" .)