Legal addendum (5)
I believe the answer is:
rider
'legal addendum' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rider that I've seen before include "Condition added to a legal document" , "Motorcyclist" , "given little weight" , "Proviso in a legal document" , "Equestrian - clause" .)