Crossword Genius

Mend a dud mobile that wasn't there originally (8)

Ross

I believe the answer is:

addendum

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'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" .)

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