If I used heroin last tues and took a UA today (Sat); would it of been cllean?

Opiates enter your blood stream easily and will accumulate in fat tissue. Drugs that remain in fat tissue will continue to enter the blood stream slowly over time. This is why you may feel the high from opiates longer than other drugs. However, this also means that your system holds on to the drug longer, but it doesn't always show up on drug tests.
The vast majority of the drug will leave your system in around six hours, if you are a heavy user for a prolonged period you might fail the test 2 weeks after you stop taking it.