Plain fish without starters in Dutch restaurant (6)
I believe the answer is:
tundra
'plain' is the definition.
(I know that tundra is a type of plain)
'fish without starters in dutch restaurant' is the wordplay.
'fish' becomes 'tuna' (tuna is a type of fish).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'starters in' indicates taking the first letters (the letters which start the words).
The first letters of 'dutch restaurant' is 'dr'.
'tuna' enclosing 'dr' is 'TUNDRA'.
(Other definitions for tundra that I've seen before include "Vast treeless plane" , "Cold northern region" , "Artic treeless plain" , "Area with frozen subsoi" , "Treeless Arctic region" .)