Assembly dismissing one shower: "Get off!" (7)

I believe the answer is:

detrain

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'get off' is the definition.
(detraining is a kind of getting off)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for detrain that I've seen before include "leave Orient Express, perhaps" , "Get off at a station" , "leave coach behind" , "get out of carriage" , "leave car" .)

I've seen this clue in The Independent.

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