Milk served over arsenic in snacks before meal (5)
I believe the answer is:
tapas
'snacks before meal' is the definition.
(type of Spanish snack)
'milk served over arsenic' is the wordplay.
'milk' becomes 'tap' (both can mean to exploit or use).
'served over' means one lot of letters go next to another (in a down clue, some letters go above others).
'arsenic' becomes 'As' (As is the chemical symbol for arsenic).
'tap'+'as'='TAPAS'
'in' acts as a link.
(Other definitions for tapas that I've seen before include "Spanish savouries" , "taste of Spain?" , "Pasta (anag) - Spanish bar food" , "Spanish eats" , "Spanish snacks" .)