One fired from Bow school head dismissed (5)

I believe the answer is:

arrow

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

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for arrow that I've seen before include "Cupid's projectile" , "Mark to indicate direction" , "Dart; pointer" , "Direction indicator" , "Archery projectile" .)

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

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