I'm just taking care of this person. Do you mind waiting a moment please?
I'm looking after this lady/gentleman. Could you give me a few minutes?
お客の対応は take care や look after を使って言うと良いですよ。
ーI'm just taking care of this person. Do you mind waiting a moment please?
「こちらの方を対応中ですので、もう少しお待ちいただけますか?」
ーI'm looking after this lady/gentleman. Could you give me a few minutes?
「こちらの女性/男性の対応中ですので、少しお待ちいただけますか?」
ご参考まで!