Boredom leads to earl's now nightly upper intake (5)
I believe the answer is:
ennui
'boredom' is the definition.
(I know that boredom can be written as ennui)
'leads to earl's now nightly upper intake' is the wordplay.
'leads to' says to take the initial letters.
The initial letters of 'earls now nightly upper intake' is 'ENNUI'.
(Other definitions for ennui that I've seen before include "boring" , "bore" , "Sense of tedium" , "Boredom, dullness" , "Feeling of disaffection" .)