Plain fish cut by doctor (6)
I believe the answer is:
tundra
'plain' is the definition.
(I know that tundra is a type of plain)
'fish cut by doctor' is the wordplay.
'fish' becomes 'tuna fish' (tuna is a type of fish).
'cut by' indicates putting letters inside (some letters cut into others).
'doctor' becomes 'dr' (abbreviation).
'tuna' going around 'dr' is 'TUNDRA'.
(Other definitions for tundra that I've seen before include "Artic treeless plain" , "Area of permanently frozen subsoil" , "Russian countryside?" , "wasteland" , "Vast treeless plane" .)