Mend a dud mobile that wasn't there originally (8)
I believe the answer is:
addendum
'mend a dud mobile that wasn't there originally' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'mend a dud mobile' is the wordplay.
'mobile' indicates anagramming the letters (mobile can mean moving).
'mend'+'a'+'dud'='mendadud'
'mendadud' with letters rearranged gives 'ADDENDUM'.
Can you help me to learn more?
(Other definitions for addendum that I've seen before include "Dud ad-men dealt with in end of book" , "Something appended as to end of book" , "Missing passage to be appended" , "Material added to the end of a book" , "Something attached to the end, say of book" .)