Initially, roasts are not common (4)
I believe the answer is:
rare
'not common' is the definition.
('rare' is the opposite to 'common')
'initially roasts are' is the wordplay.
'initially' suggests taking the first letters.
The initial letter of 'roasts' is 'r'.
'r'+'are'='RARE'
(Other definitions for rare that I've seen before include "Very infrequent, lightly cooked" , "Unusual(ly fine)" , "Special" , "(Of meat) red" , "Exceptionable" .)