Possible Clues:

• G-B-D, say
• Basic piano chord
• Eastern criminals -- one into jazz
• One in classic group of three
• Do-mi-so, for one
• Group of travellers in next answer, for example, held back in congested air traffic
• Certain musical chord
• Group of stanzas in different metres suggested by secret society
• Basic harmonic structure at the end of 'Hey Jude'
• Looking back, rail system hides one in secret Chinese organisation