It's short of breadth! (4)

I believe the answer is:

hair

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'breadth' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for hair that I've seen before include "One's crowning glory?" , "Fuzz" , "Tresses" , "Famous hirsute musical with song ` `Aquarius' ' (4)" , "Coiffure" .)

I've seen this clue in the Evening Standard.

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