Frozen plain fish, not quite dry inside (6)
I believe the answer is:
tundra
'frozen plain' is the definition.
(I've seen this before)
'fish not quite dry inside' is the wordplay.
'fish' becomes 'tuna' (tuna is a kind of fish**).
'not quite' means to remove the last letter.
'inside' indicates putting letters inside.
'dry' with its final letter removed is 'dr'.
'tuna' enclosing 'dr' is 'TUNDRA'.
(Other definitions for tundra that I've seen before include "Vast flat treeless zone" , "Vast level Arctic region with frozen subsoil" , "Vast treeless plain in the arctic regions" , "Treeless northern region" , "where you might see caribou" .)