The writer's letter has no information — it may provoke a defensive reaction (9)
I believe the answer is:
immunogen
'it may provoke a defensive reaction' is the definition.
The definition suggests a singular noun which matches the answer.
'the writer's letter has no information' is the wordplay.
'the writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'letter' becomes 'mu' (letter of the Greek alphabet).
'has' is a charade indicator (letters next to each other).
'information' becomes 'gen' (gen can informally mean information).
'im'+'mu'+'no'+'gen'='IMMUNOGEN'
Can you help me to learn more?
I've seen this clue in The Times.
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