Plant book written by Thomas (5,6)

I believe the answer is:

hardy annual

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'written by thomas' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives can occasionally mean the same thing.

'plant book' is the wordplay.
'plant' becomes 'hardy' (I can't justify this - if you can you should give a lot more credence to this answer).
'book' becomes 'annual' (annual is a kind of book).
'hardy'+'annual'='HARDY ANNUAL'

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(Other definitions for hardy annual that I've seen before include "horticultural class" , "Plant able to grow in the open all year round" .)

I've seen this clue in the Evening Standard.

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