One in rugby at large arch? (5-4)

I believe the answer is:

loose-head

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'one in rugby' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'at large arch?' is the wordplay.
'at large' becomes 'loose' (synonyms).
'arch?' becomes 'head' (synonyms).
'loose'+'head'='LOOSE-HEAD'

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I've seen this clue in the Sydney Morning Herald.

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