This Ableton tutorial covers two ways to apply swing to MIDI or audio clips using Groove files from the browser. The quickest method is to drag a Groove file directly onto a clip, which applies the feel immediately.
For more flexibility across a session, open the Groove Pool and load multiple Groove files into it. Each clip can then be assigned its own groove using the Chooser in the clip's Detail view, letting you mix and match feels without committing to a single global swing.