Tool case linking device (3,7)

I believe the answer is:

box spanner

'tool' is the definition.

'case linking device' is the wordplay.
'case' becomes 'box' (boxing is a kind of casing).
'linking device' becomes 'spanner'.
'box'+'spanner'='BOX SPANNER'

(Another definition for box spanner that I've seen is " that'll fit over his nut".)

I've seen this clue in The Guardian.

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