Bones in fish that's plain (6)
I believe the answer is:
tundra
'plain' is the definition.
(tundra is a kind of plain)
'bones in fish' is the wordplay.
'bones' becomes 'dr' (I've seen this in other clues).
'in' is an insertion indicator.
'fish' becomes 'tuna' (tuna is a type of fish).
'dr' placed into 'tuna' is 'TUNDRA'.
'that's' acts as a link.
(Other definitions for tundra that I've seen before include "Vast flat treeless zone" , "Northern permafrost region" , "Area with frozen subsoi" , "where you might see caribou" , "Russian countryside?" .)