Clancy more or less defined a new sub-genre called the techno-thriller.
His books include explanations on how the technological wizardry of military and spy-gadgets actually work.
If you read his books you'll learn a lot about how spies, the military and goverenment are supposed to work.
This genera has spread and a ton of authors have been heavily influenced by Clancy. Brad Thor, Pat Robinson, Larry Bond, Jonathan Maberry, Michael Cricton, Clive Cussler and many others write tecchno-thrillers.
Some hallmarks of his work are his scenes, transitions, time-lines and addressing technical issues and questions.
My own work has been influenced by Clancy and my most memorable praise for In the Shadow of the Dragon was it being called Clancy-esque.