rule 2.4 covers that
2.4 Ball in hand: Ball in hand in One Pocket is always behind the line, not anywhere on the table. All references to “ball in hand” within these rules are to be understood to mean “ball in hand behind the line”. When placing the cue ball, the whole cue ball must be placed above the head string line, and to play directly into an object ball, the whole object ball must be below the head string line. The edge of either ball cannot be in contact with the head string line. If either is not in a playable position then the referee or opponent warns the shooter and they must adjust and/or agree on the playable position prior to the shot being taken. If no warning is given then the shot stands. But, if the shooter ignores the warning and shoots then it is a serious foul, with opponent receiving ball in hand. ref: 6.6.2 Serious fouls