Plain fish with almost dry interior (6)
I believe the answer is:
tundra
'plain' is the definition.
(tundra is a kind of plain)
'fish with almost dry interior' is the wordplay.
'fish with' becomes 'tuna' (tuna is a type of fish. I am not sure about the 'with' bit.).
'almost' means to remove the last letter (almost the whole word is kept).
'interior' indicates putting letters inside.
'dry' with its last letter taken off is 'dr'.
'tuna' placed around 'dr' is 'TUNDRA'.
Can you help me to learn more?
(Other definitions for tundra that I've seen before include "Treeless frozen Arctic plain" , "where you might see caribou" , "Area of permafrost" , "wasteland" , "Cold plain" .)