Except you can't quite get back by tube (3)

I believe the answer is:

but

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

I don't understand how the rest of the clue works.

(Other definitions for but that I've seen before include "Nevertheless, on the other hand" , "SECOND PART OF 10" , "Except; nevertheless" , "Save" , "Only in attribution" .)

I've seen this clue in the Evening Standard.

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