Disposition of Niamh’s Glen, a UK native (10)

I believe the answer is:

englishman

I'm a little stuck... Click here to teach me more about this clue!

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

'disposition of niamh's glen a uk' is the wordplay.
'niamh's glen' anagrams to 'ENGLISHMAN'.
But, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for englishman that I've seen before include "who lives in castle?" , "E.g. Coward's idiot at 12" , "Native" , "European" , "For example, Ralph Rackstraw" .)

I've seen this clue in The Times.

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