Fast food to turn copper into pig (3,3)
I believe the answer is:
hot dog
'fast food' is the definition.
(I've seen this before)
'to turn copper into pig' is the wordplay.
'turn' indicates anagramming the letters.
'copper' becomes 'd'.
'into' means one lot of letters goes inside another.
'pig' becomes 'hog' (synonyms).
'to' anagrammed gives 'ot'.
'ot'+'d'='otd'
'otd' placed into 'hog' is 'HOT DOG'.
(Other definitions for hot dog that I've seen before include "Sausage in bread-roll for thermal canine" , "Meat in roll" , "food from a stand?" , "grub?" , "Sausage in breae roll" .)