An all-round collector of sports equipment (4,3)

I believe the answer is:

ball boy

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'sports equipment' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'an all-round collector' is the wordplay.
'an all-round' becomes 'ball' (I've seen this before).
'collector' becomes 'boy' (I am not sure about this - if you are sure you should believe this answer much more).
'ball'+'boy'='BALL BOY'

'of' is the link.

Can you help me to learn more?

(Another definition for ball boy that I've seen is " Tennis court attendant".)

I've seen this clue in the Evening Standard.

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