Definition from Wiktionary, the free dictionary
Onomatopoeia for two footsteps in succession.
- (onomatopoeia) The sound of steps on hard ground, especially of a horse's hooves.
- 1984, Anthony C. Yu, tr. 吴承恩, 西游记 (Wu Cheng'en, Journey to the West) 
- With loud clip-clops, several middle-aged women ran out from within to stare at the T’ang monk, grinning stupidly all the time.
2001, Dave Zorn, Dinky Dau: Love, War, and the Corps:
- Suddenly I heard the familiar “clip-clop, clip-clop” of shower shoes coming up the path behind me—and a rapid pace it was.
- 2003, Ann Redisch Stampler, Something for Nothing 
- All night long, Dog heard the clip-clop of horses hauling the wagons to market.
- To make a clip-clop noise.
- Used particularly of horses.
- Often repeated one or more times in succession to indicate multiple or continued steps.