American

a dark horse - a person who wins a race or competition although no one expected them to.

She's a dark-horse candidate for the position of company director. (always before noun)

• a feather in someone's cap
• a feather in your cap
• a feeding frenzy
• a feel for
• a feel for something
• a femme fatale
• a fender bender
• a few fries short of a Happy Meal
• a fifth wheel
• a fifty-fifty chance
• a fight for life
• a fighting chance
• a fig leaf
• a figment of imagination
• a figment of your imagination
• a figure of fun
• a fine kettle of fish
• a fine line
• a finger in every pie
• a firm hand on the tiller
• a fishing expedition
• a fish out of water
• a flash in the pan
• a flight of fancy
• a flight of fantasy