Crossword Genius

Grateful nation buries partner in arms? (4)

Ross

I believe the answer is:

ulna

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'arms?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'grateful nation buries partner' is the wordplay.
'ULNA' is hidden in amongst the letters.
However, I'm uncertain how the hidden word is indicated.

'in' is the link.

Can you help me to learn more?

(Other definitions for ulna that I've seen before include "part of arm" , "Car document; ticket book" , "One in arms" , "The longer and inner bone in forearm" , "Part of body" .)

I've seen this clue in The Telegraph.
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