Possible Clues:

• Driving force, impetus
• It doesn't slow down for a split second to express hesitation
• Corbynites having impetus
• What mover has importance? Greek character on the up
• Labour's left having impetus
• Impetus gained by a moving object
• Impetus bringing hesitation after a little while
• Energy shown by second Greek character turning up
• Weight and rate of progress of male fatty tissue
• Hesitation following second thrust