clone during flowering?

In order to get a clone off of a plant that is already flowering would you have to put the whole plant back in to veging after harvesting it and then get one or can you take a clipping off the flowering plant and put that back in to the longer light cycle?

Either or. I'd grab a couple lower ones off now, just in case the re-veg fails for some unexpected reason. It's best done before flowering. Depends on how far in you are as to how long it'll take to root . . . always longer in flower.
I suppose a grower could be too far along to take clones that didn't have fair sized pop-corn on them, though . . . there should be some measly growth under your canopy somewhere?