Storm is incorrect, you can delay outbursts; revenge doesn't consume Outburst, so you can delay sending it by spending a few GCDs. You can send outburst on Clap only if your only intention is damage and you're in real big pull hours. I wouldn't personally think of intentionally doing it unless there's at least like 10, 12 mobs. It only even starts to get close to the raw damage of an outbursted Shield Slam at about 8 targets, and it gives drastically less rage.
In ST or 2 targets, you'd want to send Outburst on Shield Slam; 2 targets have the damage close enough between Blast and Shield Slam that the rage will generally win out. If Shield Slam isn't available, you can gamble some time on resets by pressing Revenge for a few globals. This is fairly uncommon with current Thane with how many Shield Slam and Thunder Blast resets we get, but it does still sometimes happen.
In 3+ Targets, you want to send Outburst on Thunder Blast if it's available, otherwise Shield Slam. You can also delay here if Blast is on CD outside of Avatar or if Avatar is almost off CD to give you more charges of Thunder Blast by pressing Revenge for a few globals again.
And if you have an ionizing strike proc, ignore the above and just send that Thunder Blast; if it procs and you have ionizing, it's generally close to shield slam levels of damage if not higher, so if an outburst happens to exist you can just send it.