Lean on pupil occasionally showing boredom (5)
I believe the answer is:
ennui
'boredom' is the definition.
(I know that boredom can be written as ennui)
'lean on pupil occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'lean'+'on'+'pupil'='leanonpupil'
The alternate letters of 'leanonpupil' are 'ENNUI'.
'showing' acts as a link.
(Other definitions for ennui that I've seen before include "Feeling when tired of life?" , "Lack of interest" , "French boredom and lassitude" , "Mental weariness" , "Weary feeling" .)