One enters level two (4)
I believe the answer is:
pair
'two' is the definition.
(pair is a kind of two)
'one enters level' is the wordplay.
'one' becomes 'i' (Roman numeral).
'enters' means one lot of letters goes inside another.
'level' becomes 'par' (I have seen 'Accepted level ' mean 'par' so perhaps 'level' could also mean 'par'**).
'i' inserted inside 'par' is 'PAIR'.
(Other definitions for pair that I've seen before include "Twosome" , "Husband and wife" , "Two ducks in one game of cricket" , "Agreement to avoid division" , "Brace, couple" .)