Band beginning to roast slice of beef outside (6)
I believe the answer is:
Here is my best explanation:
'band' is the definition.
'beginning to roast slice of beef outside' is the subsidiary indication.
'beginning to' indicates taking the first letters.
'slice of beef' becomes 'steak'.
'outside' indicates putting letters inside.
The initial letter of 'roast' is 'r'.
'r' inserted 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".)