Salt water or nitrogen in cheese (5)
I believe the answer is:
brine
'salt water' is the definition.
(brine is water containing salt)
'nitrogen in cheese' is the wordplay.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'in' means one lot of letters goes inside another.
'cheese' becomes 'brie' (Brie is an example).
'n' put into 'brie' is 'BRINE'.
'or' acts as a link.
(Other definitions for brine that I've seen before include "Sea water" , "Salty water" , "pickle" , "Seawater" , "Strong salt-water solution" .)