Possible Clues:

• Counting-rhyme start
• Word in a counting-down rhyme
• It stands for 1
• "___, meeny, miney, moe..."
• Chooser's nonsense word
• Start of a count-off
• '___, meeny, miney, moe...'
• Count-out start
• One of a few 'choice' parts
• and 35 across, Mo is a follower of this trio