Be the right one inside with the cheese (4)
I believe the answer is:
brie
'cheese' is the definition.
(I know that Brie is a type of cheese)
'be the right one inside' is the wordplay.
'the right' becomes 'r' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'inside' means one lot of letters goes inside another.
'r'+'i'='ri'
'be' enclosing 'ri' is 'BRIE'.
'with the' acts as a link.
(Other definitions for brie that I've seen before include "Soft and creamy mild white cheese" , "dairy product" , "Round soft Fr. cheese" , "Soft, creamy French cheese" , "cheese-producing region" .)