Timothy Leary Phrase, "turn On, __, Drop Out"

This page is the CodyCross answer for the clue below from Space Exploration, Group 920, Puzzle 1 (Hydrogen and Helium).

Question is: Timothy Leary Phrase, "turn On, __, Drop Out"

and the answer is: TUNEIN

Word Meaning:

TUNE IN — (transitive) To select a channel, station, etc., as on television or radio.

Definitions from Wiktionary.