Crossword Genius

Growing evidence of when to write? (5)

Ross

I believe the answer is:

aspen

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'growing evidence' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

'when to write?' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'to write?' becomes 'pen' (synonyms).
'as'+'pen'='ASPEN'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for aspen that I've seen before include "Trembling poplar tree" , "quill-like?" , "somewhere in Colorado" , "Tree, kind of poplar" , "Type of poplar" .)

I've seen this clue in the Evening Standard.
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