Ichidan/ru-verb

1. to be clear (of a sight, sound, colour, etc.); to be bright; to be vivid; to be crisp
2. to be clear-headed; to be alert; to be on the ball; to be wide awake
3. to look upbeat; to be perky; to be cheerful (often as 冴えない)
4. to master (a skill); to excel at; to cleanly execute
5. to be satisfying (often as 冴えない)
6. to become frigid; to become intensely cold (original meaning; now somewhat old-fashioned)