"To start a combo" means to initiate a sequence of actions or moves, usually in a video game or combat sport, where one action leads to another smoothly and quickly for a continuous attack or series of moves. Full definition
A starter has to be at the start of a combo — you can't use it if you already started your combo with something else. (sirlin.net)
Once you have some breathing room, start a combo of your own to land a few hits on your opponent. (techradar.com)
A combination of light punch, light kick, heavy punch and heavy kick is used to start a combo then further action can be performed by juggling the opponent in the air. (gearnuke.com)