One thing an umpire may do is to read out a list (4-4)

'umpire may do is to read out a list' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'one thing an' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'an' could be 'a' and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?