Local food served up in Hamburg (4)
I believe the answer is:
grub
'up in hamburg' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'local food' is the wordplay.
'local' becomes 'gru' (I can't explain this - if you can you should believe this answer much more).
'food' becomes 'B' (B is an example).
'gru'+'b'='GRUB'
'served' acts as a link.
Can you help me to learn more?
(Other definitions for grub that I've seen before include "food, commonly" , "Table needs" , "Larva; dig shallowly" , "Common food" , "Nosh - larva" .)