Plain fish almost dry inside (6)
I believe the answer is:
tundra
'plain' is the definition.
(tundra is a kind of plain)
'fish almost dry inside' is the wordplay.
'fish' becomes 'tuna' (tuna is a kind of fish**).
'almost' means to remove the last letter (most of the word but not all of it).
'inside' is an insertion indicator.
'dry' with its last letter taken away is 'dr'.
'tuna' going around 'dr' is 'TUNDRA'.
(Other definitions for tundra that I've seen before include "Cold plain" , "Vast treeless plane" , "Arctic plain with permafrost" , "Arctic region of permafrost" , "Russian countryside?" .)