Ought to replace lead in unplanned Western (10)

I believe the answer is:

occidental

'western' is the definition.
('occidental' can be similar in meaning to 'western')

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

(Other definitions for occidental that I've seen before include "Westerner" , "Western" , "Relating to the West" .)

I've seen this clue in The Independent.

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