Hi, Dylan, the definition is “UCLA freshers at party”, where UCLA stands for a generic US university. For the wordplay, you have a three letter word for “take drugs”, with “sex, not without”, all of this “inhaling” the chemical symbol for helium.
The three word phrase sums up the theme nicely. Does it refer to something we have already done, or is it something we have to do? As the phrase “appears in the completed grid”, I think this rules out the latter.