Possible Clues:

• Usual "SNL" start
• Sketch that kicks off "SNL" ... or a phonetic hint to the start of 18-, 29-, 37- or 49-Across
• "Saturday Night Live" start - both unwelcoming and welcoming
• Start of each 'S.N.L.' episode ... or a hint to the initials of the words in 17-, 24-, 35- and 47-Across
• Segment before the title sequence
• Typical 'S.N.L.' start