An inside straight draw: 5-6-X-8-9.

holding 4 cards to a straight but requiring only one rank to make it eg holding 3,4,5,7 and needing a 6

Aka, inside straight draw. A straight draw that can only be made with 1 card instead of two (for instance, 5689 is a bellybuster straight draw to the 7). See also: Double Bellybuster, Open Ended.