What a pin could cause (4)

I believe the answer is:

pain

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'cause' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'what a pin' is the wordplay.
'a pin' anagrams to 'PAIN'.
But, I am unsure how this is indicated.

'could' acts as a link.

Can you help me to learn more?

(Other definitions for pain that I've seen before include "Suffering of body or mind" , "Sensation of hurt" , "Hurt feeling" , "Bodily suffering or mental anguish" , "Sensation of acute discomfort" .)

I've seen this clue in the Evening Standard.

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