Some scribes omitted sweeper (5)
I believe the answer is:
besom
'sweeper' is the definition.
(I've seen this before)
'some scribes omitted' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'BESOM' is hidden in the letters of 'scribes omitted'.
(Other definitions for besom that I've seen before include "Sweeping item" , "Broom made from twigs tied to a handle" , "Twig broom" , "sweeper" , "Twig-broom" .)