Crossword Genius

Is afraid a thousand choirboys have got trapped (8)

Ross

I believe the answer is:

trembles

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'is' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for trembles that I've seen before include "Quivers" , "Shivers involuntarily as from fear" .)

I've seen this clue in the Evening Standard.
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