Boredom reining us in from time to time (5)
I believe the answer is:
ennui
'boredom' is the definition.
(I know that boredom can be written as ennui)
'reining us in from time to time' is the wordplay.
'from time to time' means one should take alternating letters (letters removed on a regular basis).
'reining'+'us'+'in'='reiningusin'
The alternating letters of 'reiningusin' are 'ENNUI'.
(Other definitions for ennui that I've seen before include "Feeling of disaffection" , "Listless boredom" , "French boredom and lassitude" , "Boredom, listlessness" , "Feeling when tired of life?" .)