Crossword Genius

How the ballboy gets when he puts on weight? (7)

Ross

I believe the answer is:

rounder

I'm a little stuck... Click here to teach me more about this clue!

'how the ballboy gets' is the definition.
I can't judge whether this definition defines the answer.

'he puts on weight?' is the wordplay.
I cannot really understand how this works, but
'he' could be 'der' and 'der' is found in the answer.
This explanation may well be incorrect...

'when' acts as a link.

Can you help me to learn more?

(Other definitions for rounder that I've seen before include "More circular" , "complete circuit in a game" , "More spherical" , "More rotund" , "Or under (anag.)" .)

I've seen this clue in the Evening Standard.
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