Sodium, carbon, hydrogen and oxygen in snack (5)
I believe the answer is:
nacho
'snack' is the definition.
(I know that nacho is a type of tortilla chip)
'sodium carbon hydrogen and oxygen' is the wordplay.
'sodium' becomes 'NA' (chemical symbol).
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'and' says to put letters next to each other.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'na'+'c'+'h'+'o'='NACHO'
'in' acts as a link.
(Other definitions for nacho that I've seen before include "Mexican snack item" , "Mexican dish" , "Mex. corn chip" , "kind of chip" , "Mexican corn chip eaten with cheese, etc" .)