Band beginning to roast slice of beef outside (6)
I believe the answer is:
streak
'band' is the definition.
(I have seen 'Long band' mean 'streak' so perhaps 'band' could also mean 'streak')
'beginning to roast slice of beef outside' is the wordplay.
'beginning to' suggests taking the first letters.
'slice of beef' becomes 'steak'.
'outside' is an insertion indicator.
The first letter of 'roast' is 'r'.
'r' placed into 'steak' is 'STREAK'.
(Other definitions for streak that I've seen before include "Go faster" , "Spell (of good luck)" , "Skater (anag.)" , "Run naked as stunt" , "Run naked; long mark" .)