No lad's sure a write-up is defamatory (10)

I believe the answer is:

slanderous

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'is defamatory' is the definition.
(synonyms)

'no lad's sure a write-up' is the wordplay.
'no lad's sure' anagrams to 'SLANDEROUS'.
But, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for slanderous that I've seen before include "Done as slur (anag)" , "Malicious" , "Defamatory" , "smear" .)

I've seen this clue in The Irish Independent.

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