Possible Clues:

• Give an undertaking
• "Cross your heart?"
• Hopeful sign as note is in no fancy language
• Declaration sometimes made with crossed fingers behind the back
• Verbal or other commitment of one person to another
• Pledge - my word!
• 'You swear?'
• 'Pardon me!', I asked, regularly breaking vow
• Pinkie swear, e.g.
• Commit to a little coastal walk with one's wife -- finally!