Having switched sides, slavishly follow line (4)

I believe the answer is:

oche

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

I can't explain the remainder of the clue.

(Other definitions for oche that I've seen before include "What darts player stands behind" , "Line of 25 [ARROEWS]" , "The mark at darts" , "Dart player's mark" , "Line from which dart-players throw" .)

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