Possible Clues:

• It grows beneath the surface
• Ultimate source
• Word sung three times before 'for the home team' in 'Take Me Out to the Ball Game'
• Finishes letter to go out to part of underground structure
• Part of a hair
• Unrestricted user account on a computer
• Anchor that may be square?
• Dig around
• Part of a tree or bush
• Basic cause or origin