Of ball ammunition, singularly? (5)

I believe the answer is:

round

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'of ball' is the definition.
'round' can be an answer for 'ball' (thesaurus). I am not sure about the 'of' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for round that I've seen before include "Approximate" , "regular series" , "Piece of boxing" , "purchase of drinks" , "Set of drinks - set of holes" .)

I've seen this clue in the Evening Standard.

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