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.
Maybe you can see a link between them that I can't see?

'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' put after 'agent' is 'AGENTIVE'.

'in' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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