A feeling of lack of interest or energy (7)

I believe the answer is:

languor

(Other definitions for languor that I've seen before include "Indolence" , "Apathy" , "Mental laziness" , "'Lassitude, lethargy (7)'" , "Feeling of boredom or feebleness" .)

I've seen this clue in The Irish Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.