Sleuth casing Long Island firm (5)

I believe the answer is:

telic

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'firm' is the definition.
Both the answer and definition are adjectives.
Perhaps they are linked in a way I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for telic that I've seen is " (Of action) directed to a definite end".)

I've seen this clue in The Sunday Times and The Times.

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