Rearrange a space as in inland lake (7,3)
I believe the answer is:
caspian sea
'inland lake' is the definition.
(the Caspian Sea is sometimes considered a lake)
'rearrange a space as in' is the wordplay.
'rearrange' indicates anagramming the letters (I've seen this in other clues).
'a'+'space'+'as'+'in'='aspaceasin'
'aspaceasin' is an anagram of '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" , "World's largest lake" , "There's lots of water here" , "Largest salt lake in the world" , "Lake bordered by both Russia and Iran" .)