German after old Mexican food (5)
I believe the answer is:
nacho
'mexican food' is the definition.
(I've seen this before)
'german after old' is the wordplay.
'german after' becomes 'nach' ('after' in German).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'nach'+'o'='NACHO'
(Other definitions for nacho that I've seen before include "Tortilla with cheese, etc" , "Small piece of tortilla with a topping" , "Mex. corn chip" , "Hot snack" , "Mexican food item" .)