Look into it earnestly and find a clear answer that you can give to the customer.
「お客様に説明できる明確な回答を下さい」を直訳したら「find a clear answer that you can explain to the customer」になりますが、そうすると「explain an answer」という形になって、不自然な英語だと思います。「explain」のかわりに「give」にしました。
Please investigate the matter thoroughly so I can provide a clear answer to our customers.
It's important that we offer our customers complete information, so please reply after examining this issue closely.
In order to give our customers the proper information, I need you to take a serious look at this and get back to me.
上記の英語で使われている単語・フレーズを確認しましょう。
・真剣に調査する investigate XX thoroughly, examine XX closely, take a serious look at XX
・客に明確に回答する provide a clear answer to one's customers, offer one's customers complete information, give one's customers the proper information
アメリカ合衆国 
日本 
