Fish found round mostly arid Arctic plain (6)
I believe the answer is:
'arctic plain' is the definition.
(type of geographical region)
'fish found round mostly arid' is the wordplay.
'fish' becomes 'tuna' (tuna is a type of fish).
'found round' is an insertion indicator.
'mostly' means to remove the last letter.
'arid' becomes 'dry' (similar in meaning).
'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 level Arctic region with frozen subsoil" , "Vast flat treeless zone" , "Treeless northern region" , "Area of permafrost" .)