What does waiting/clock icon next to a message mean and what if I delete message in this state?
The Clock icon means that you haven't sent the message yet. If you see a single tick mark it means that you have sent the message and it has been received by the WhatsApp server(but not the destined user)
If you see a double tick mark, that means the message has been received by the recipient too.

If a message has a single tick with it, it can't be stopped from getting to the user. But if you sent a message when offline and deleted when the phone was offline I don't think the message will reach its destination.