Argon, say, is great after mixing around nitrogen (5,3)
I believe the answer is:
inert gas
'argon say' is the definition.
(I know that Ar is a type of inert gas)
'is great after mixing around nitrogen' is the wordplay.
'after mixing' is an anagram indicator.
'around' means one lot of letters goes inside another.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'is'+'great'='isgreat'
'isgreat' with letters rearranged gives 'iertgas'.
'iertgas' going around 'n' is 'INERT GAS'.
(Other definitions for inert gas that I've seen before include "Passive element in the helium group" , "one at the edge of a table" , "He, amongst others" , "One expects no reaction from such" , "Eg, helium, xenon etc." .)