Transport system sometimes put on test (4)

I believe the answer is:

tube

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'transport system' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for tube that I've seen before include "Pipe - railway" , "Pipe; underground train" , "TV" , "London Underground or long cylinder" , "Pipe or hose" .)

I've seen this clue in the Evening Standard.

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