Processed meat gone with butter by one (8)
I believe the answer is:
pastrami
'processed meat' is the definition.
'gone with butter by one' is the wordplay.
'gone with' means one lot of letters go next to another.
'butter' becomes 'ram' (a ram is something which 'butts').
'by' becomes 'past' (synonyms).
'one' becomes 'i' (Roman numeral).
'ram' after 'past' is 'pastram'.
'pastram'+'i'='PASTRAMI'
(Other definitions for pastrami that I've seen before include "Thinly sliced beef" , "Highly seasoned cut of smoked beef" , "Seasoned smoked beef, served in thin slices" , "Spiced smoked beef" , "smoked joint" .)