Doesn't raise head above parapet in castles in Ulster (5,4)

I believe the answer is:

keeps down

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'castles in ulster' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how they can define each other.

I cannot understand how the rest of the clue works.

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I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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