Crossword Genius

Instrument I have in a case (8)

Ross

I believe the answer is:

agentive

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

'instrument i have' is the wordplay.
'instrument' becomes 'agent' (I've seen this before).
'i have' becomes 'I've' (contraction of 'I have').
'agent'+'ive'='AGENTIVE'

'in' is the link.

Can you help me to learn more?

I've seen this clue in The Sunday Times.
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