Plain fish, mostly dry on the inside (6)
I believe the answer is:
tundra
'plain' is the definition.
(I know that tundra is a type of plain)
'fish mostly dry on the inside' is the wordplay.
'fish' becomes 'tuna' (tuna is a type of fish).
'mostly' means to remove the last letter.
'on the inside' means one lot of letters goes inside another.
'dry' with its final letter taken off is 'dr'.
'tuna' enclosing 'dr' is 'TUNDRA'.
(Other definitions for tundra that I've seen before include "Vast flat treeless zone" , "Vast level Arctic regions with frozen subsoil" , "Area of permanently frozen subsoil" , "Vast treeless plane" , "Treeless Arctic region" .)