This was a flashback scene. If you recall in a scene or two before Cliff starts repairing the antenna, Cliff was told that a specific stunt coordinator, played by Kurt Russell, was coordinating for the show Lancer that Rick was currently acting in so the likelihood of Cliff being allowed to be Rick's stunt double was practically impossible. The audience is left wondering why until Cliff ponders back and we find out that in a previous production with the stunt coordinator on The Green Hornet Cliff threw Bruce Lee into the coordinator's wife's car. Cliff was already on thin ice as the wife absolutely hated Cliff because he
allegedly murdered his wife and the coordinator's wife didn't trust him.
So when the car got damaged, Cliff was given the boot. Cliff then agrees that he deserved getting canned.
There has been some disagreement among fans regarding if this was a flashback or a daydream, to the point that the Wikipedia article for the movie got edited a couple of times stating it was one or the other. Some think that it is a daydream and that Cliff embellished his skills fighting Bruce Lee. However, in the end Cliff does get fired which, to me at least, reads as a flashback. If one daydreams, usually one imagines coming out on top and does not resign themselves with a "Yeah I deserved that..." sentiment.