This trembles when shut in (5)
I believe the answer is:
aspen
'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" .)