Each head's scratched fiercely (4)

I believe the answer is:

very

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'each' 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?

(Other definitions for very that I've seen before include "Extremely - light" , "To a large extent" , "light, perhaps" , "Exactly, really" , "Absolutely" .)

I've seen this clue in The Independent.

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