Former source of mutton I perceived as form of beef (8)
I believe the answer is:
pastrami
'form of beef' is the definition.
'former source of mutton i' is the wordplay.
'former' becomes 'past' (similar in meaning).
'source of mutton' becomes 'ram' (a ram's flesh might be used as mutton).
'past'+'ram'+'i'='PASTRAMI'
'perceived as' is the link.
(Other definitions for pastrami that I've seen before include "Sliced cured beef" , "beef dish" , "Highly seasoned cut of smoked beef" , "Spicy smoked beef" , "Thinly sliced beef" .)