Letter or manuscript useful to gardeners (5)

I believe the answer is:

corms

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'useful to gardeners' is the definition.
I can't judge whether this defines the answer.

'letter or manuscript' is the wordplay.
'letter' becomes 'c' ().
'manuscript' becomes 'ms' (abbreviation for manuscript).
'c'+'or'+'ms'='CORMS'

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I've seen this clue in the Evening Standard.

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