Supporter of bypass has final drink (3,3,3,4)

I believe the answer is:

one for the road

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

I can't explain the remainder of the clue.

(Other definitions for one for the road that I've seen before include "alcoholic drink" , "not for rail traveller?" , "Individual presumably against railways" , "Last drink before leaving" , "driver?" .)

I've seen this clue in The Daily Express.

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