Band beginning to roast slice of beef outside (6)
I believe the answer is:
Here is my best explanation:
'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 "Irregular mark - run naked", "Run nude like skater", "Long band", "Run naughtily naked", "Distinctive characteristic", "Nude dash", "Move very quickly", "Characteristic", "Flash of colour - run naked" and "Run of luck".)