Possible Clues:

• Requiring great effort
• Like some tasks assigned to Poirot?
• The fellow managed to include ridiculous clue, fiendishly difficult
• Powerful nuclear process depending on helium
• Enormously demanding (task)
• Does this make her strong enough to see you have an inclination?
• So enormous a task for Poirot?
• Energy evident in revised re-launch of Old Labour
• Formidable, as a task
• Man takes nuclear option requiring very hard work