To write 'Shaun' or 'Kathleen', for example, would be a terrible sin in Gaelic (9)

I believe the answer is:

anglicise

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'to write' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.

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

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I've seen this clue in The Irish Times.

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