Crossword Genius

Wouldn't part with sole weapon that's proved trustworthy (4,4,4)

Ross

I believe the answer is:

kept ones word

'proved trustworthy' is the definition.
(kept a promise)

'wouldn't part with sole weapon' is the wordplay.
'wouldn't part with' becomes 'kept' (kept hold of).
'sole' becomes 'one' (synonyms).
'weapon' becomes 'sword' (sword is a kind of weapon).
'kept'+'one'+'sword'='KEPT ONES WORD'

'that's' is the link.

I've seen this clue in The Guardian.
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