The blahs (5)
I believe the answer is:
ennui
'the blahs' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ennui that I've seen before include "Lack of interest" , "Mental weariness" , "Boredom in France" , "Effect of boredom" , "What's tedious and instils such listlessness" .)