A good round belt-fastener used as a sling (9)

I believe the answer is:

parbuckle

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'a sling' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'a good round belt-fastener' is the wordplay.
'a good round' becomes 'par' (I am not sure about this - if you are sure you should believe this answer much more).
'belt fastener' becomes 'buckle' (I've seen this before).
'par'+'buckle'='PARBUCKLE'

'used as' is the link.

Can you help me to learn more?

(Another definition for parbuckle that I've seen is " Rope sling".)

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

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