Smoked beef I topped with old butter (8)
I believe the answer is:
pastrami
'smoked beef' is the definition.
(I've seen this before)
'i topped with old butter' is the wordplay.
'topped with' means one lot of letters go next to another.
'old' becomes 'past' ('past' can be similar in meaning to 'old').
'butter' becomes 'ram' (I've seen this before).
'past'+'ram'='pastram'
'i' after 'pastram' is 'PASTRAMI'.
(Other definitions for pastrami that I've seen before include "Thinly sliced beef" , "smoked joint" , "A tramp is (anag) - meat" , "beef dish" , "Seasoned dish" .)