An emu three daughters trained for rider (8)
I believe the answer is:
addendum
'for rider' is the definition.
(thesaurus)
'an emu three daughters trained' is the wordplay.
'three daughters' means three replacements for 'daughters'.
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'trained' indicates an anagram (to train can mean to grow something into a shape).
'an'+'emu'+'d'+'d'+'d'='anemuddd'
'anemuddd' with letters rearranged gives 'ADDENDUM'.
(Other definitions for addendum that I've seen before include "Text added at the end" , "Missing passage to be appended" , "Additional material at the end" , "Supplementary item in book, magazine etc" , "Piece added on" .)