Run of luck (6)
I believe the answer is:
streak
'run of luck' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for streak that I've seen before include "Stripe of contrasting colour" , "'Sudden flash, of lightning say (6)'" , "Run naked as stunt" , "trace?" , "Skater (anag.)" .)