Fish doctor’s intervention is plain (6)
I believe the answer is:
tundra
'is plain' is the definition.
(I've seen this before)
'fish doctor's intervention' is the wordplay.
I cannot quite understand how this works, but
'doctor' could be 'dr' (synonyms) and 'dr' is found in the answer.
'fish' could be 'tuna' (tuna is a type of fish) and 'tuna' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tundra that I've seen before include "Area of permanently frozen subsoil" , "wasteland" , "Artic treeless plain" , "Arctic region of permafrost" , "Vast area" .)