Attacks just as one starts (4,5)

'starts' is the definition.
('set about' can be a synonym of 'start')

'attacks just as one' is the wordplay.
I cannot really see how this works, but
'attacks' could be 'sets' (setting is a kind of attacking) and 'sets' is found in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
The remaining letters 'bout' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?