It is the custom for presidential nominees of the Democratic and Republican parties to get security briefings from members of the intelligence agencies, presumably so that they have a better awareness of foreign policy issues and are better prepared when one of them eventually takes office. They do not have to get security clearances for this purpose. This briefing process is not required by law but has evolved over time and has been uncontroversial.
This year, things are different. The controversy over Hillary Clinton using her private email server to send official correspondence some of which were classified has resulted in the Speaker of the House of Representatives Paul Ryan suggesting that she could not be trusted to receive those briefings. Meanwhile, some intelligence professionals are uneasy about briefing Donald Trump, though the Director of National Intelligence James Clapper has said that he will be briefed.
Interestingly, all members of Congress are exempt from having to get security clearances before receiving intelligence briefings, the idea being that the people have entrusted them with state secrets by voting them into office. Those who deal with classified information can be required to take an oath of confidentiality.
Staffers on sensitive Congressional committees can get security clearances but those who work in the offices of for the members cannot. Cabinet members need to get security clearances, so presumably Clinton got one before she became Secretary of State.