Rotary file (4)
I believe the answer is:
'rotary file' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for burr that I've seen before include "Prickly, clinging seed-case" , "Rough pronunciation of the letter r as in some areas" , "trouble" , "Former US vice-president - dialect way of pronouncing R" , "Clinging seed-case; whetstone" .)