Plain fish eating doctor (6)
I believe the answer is:
tundra
'plain' is the definition.
(tundra is a kind of plain)
'fish eating doctor' is the wordplay.
'fish' becomes 'tuna' (tuna is a kind of fish**).
'eating' indicates putting letters inside.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'tuna' enclosing 'dr' is 'TUNDRA'.
(Other definitions for tundra that I've seen before include "northern waste" , "where you might see caribou" , "Area of permafrost" , "Treeless Arctic region" , "Vast level Arctic regions with frozen subsoil" .)