Possible Clues:

• Warmth of a river — one in Portugal mostly
• Keenness more difficult for a Cockney, do we hear?
• Passion expressing how you must try to improve
• Passion of artist turning grim
• Heat that could make a run grim
• Passion is more intense, commonly overheard
• Passion more difficult, it's commonly said?
• Fire starts off difficult time
• Enthusiasm is more exhausting, as Cockneys may say
• Passion that's reportedly more difficult for EastEnders