Promises but donates only a single weapon (5,4,4)

I believe the answer is:

gives ones word

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'promises' is the definition.
I know nothing about this answer so I can't tell whether this works.

'donates only a single weapon' is the wordplay.
'donates' becomes 'gives' (I've seen this before).
'only' becomes 'one' (synonyms).
'a single weapon' becomes 'sword' (sword is a kind of weapon. I am not sure about the 'single' bit.).
'gives'+'one'+'sword'='GIVES ONES WORD'

'but' acts as a link.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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