High quality silver robbery reported (1-5)

I believe the answer is:

a-grade

'high quality' is the definition.

'silver robbery reported' is the wordplay.
I cannot really see how this works, but
'silver' could be 'ag' ('Ag' can be a synonym of 'silver') and 'ag' is found within the answer.
This may be the basis of clue (or it may be nonsense).

I've seen this clue in The Independent.

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