The boat behind gives way to the boat ahead. Clear astern means the whole of your hull and equipment in normal position is behind a line square across the aftermost point of the boat ahead.
The boat astern must be able to avoid the boat ahead whatever reasonable course the leader sails, including slowing down. Sailing into the back of a boat that has eased her sheets or luffed slightly is a breach by the boat astern, subject to rule 16 if the leader changed course sharply.
The moment the boat astern gets her bow past that line she becomes overlapped and rule 11 decides who keeps clear. If she becomes overlapped to leeward, rule 15 means she must at first give the new windward boat room to keep clear, and if the overlap begins within two of her hull lengths, rule 17 limits how high she can sail.
Common situations
Surfing downwind
Downwind, a boat clear astern often closes fast on a wave. She is still the keep-clear boat until an overlap begins. Steer to miss the leader's stern by a boat width and a half, not by inches.