Figure exercise is a bore (7,5)

I believe the answer is:

diamond drill

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'a bore' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?

'figure exercise' is the wordplay.
'figure' becomes 'diamond' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'exercise' becomes 'drill' (military exercise).
'diamond'+'drill'='DIAMOND-DRILL'

'is' is the link.

Can you help me to learn more?

I've seen this clue in The Daily Express.

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