Lassitude seen in pupil regularly (5)
I believe the answer is:
ennui
'lassitude' is the definition.
(both can mean a feeling of weariness)
'seen in pupil regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'seen'+'in'+'pupil'='seeninpupil'
The alternate letters of 'seeninpupil' are 'ENNUI'.
(Other definitions for ennui that I've seen before include "tiredness" , "Result of boredom" , "to make one list less when connected" , "Feeling of weariness or boredom" , "Boredom or discontent" .)