Changed seat, sure to be going in one direction (9)

I believe the answer is:

eastbound

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'going in one direction' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?

'changed seat sure' is the wordplay.
'changed' indicates anagramming the letters.
'sure' becomes 'bound' (synonyms).
'seat' anagrammed gives 'east'.
'east'+'bound'='EASTBOUND'

'to be' is the link.

Can you help me to learn more?

(Other definitions for eastbound that I've seen before include "as in A Passage to India?" , "Moving toward the orient" , "heading for the orient" .)

I've seen this clue in The Daily Express.

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