a person who you had a romantic relationship with in the past.

I bumped into an old flame of yours in Oxford on Saturday.

• a fait accompli
• a fallen angel
• a fallen idol
• a fallen woman
• a fall from grace
• a fall guy
• a false alarm
• a false dawn
• a false economy
• a false start
• a family man
• a far cry from
• a far cry from something
• a fashion victim
• a fast talker
• a fast track
• a fat cat
• a fate worse than death
• a fat lot of good
• a fat lot of use
• a faux pas
• a favourite son
• a feather in cap
• a feather in someone's cap
• a feather in your cap