Crossword Genius

This trembles when shut in (5)

Ross

I believe the answer is:

aspen

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'this trembles' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'when shut in' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'shut in' becomes 'pen' (I've seen this in another clue).
'as'+'pen'='ASPEN'

Can you help me to learn more?

(Other definitions for aspen that I've seen before include "Its leaves flutter in the slightest breeze" , "Tree that quivers" , "Colorado winter sports town" , "winter resort" , "Colorado ski resort - tree" .)

I've seen this clue in The Independent.
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