The Federal Communications Commission postponed a plan Monday to "clarify" its April crackdown on broadcasts of indecent or obscene material claiming it needs more time to study First Amendment concerns. The agency was scheduled to act today on petitions from the National Assn. of Broadcasters and others who criticized the policy statement as too vague and confusing. The FCC crackdown on on-air indecency arose from complaints against three stations, including Pacifica station KPFK-FM in Los Angeles. The FCC sent warning letters to the stations but Pacifica later appealed the new standards in federal court, saying the new rules violated the First Amendment.