I have to follow spy, in case (8)

I believe the answer is:

agentive

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

'i have to follow spy' is the wordplay.
'i have' becomes 'I've' (contraction of 'I have').
'to follow' is a charade indicator (letters next to each other) (some letters go after or follow others).
'spy' becomes 'agent' (secret agent).
'ive' after 'agent' is 'AGENTIVE'.

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in The Independent.

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