Human taking time in lesson (6)
I believe the answer is:
mortal
'human' is the definition.
(synonyms)
'time in lesson' is the wordplay.
'time' becomes 't' (abbreviation).
'in' is an insertion indicator.
'lesson' becomes 'moral' (as in the moral of the story).
't' going inside 'moral' is 'MORTAL'.
'taking' is the link.
(Other definitions for mortal that I've seen before include "Certain to die" , "Human - transitory" , "liable to die" , "Human being, subject to death" , "Liable to cause death" .)