Secretive person, quiet but inwardly upset (4)

I believe the answer is:

clam

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'upset' is the definition.
The answer and definition can be both to do with contact as well as being verbs in their base form.
Maybe there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for clam that I've seen before include "Shellfish - secretive type" , "reticent person" , "Quahog" , "Bi-valve with firmly-closed shell" , "shy person" .)

I've seen this clue in The Independent.

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