Possible Clues:

• The cockney has not got one over the eight to make him so high (6)
• Distance form top to bottom
• Altitude or elevation
• Distance from ground
• Henry joins racing crew to gain stature
• H over one under the start of 31 across up there
• H is one up and over the eight
• Vertical extension
• Rising ground; extreme example
• Dimension from top to bottom