Is afraid a thousand choirboys have got trapped (8)
I believe the answer is:
trembles
'is' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for trembles that I've seen before include "Quivers" , "Shivers involuntarily as from fear" .)