Fatal lesson about time (6)
I believe the answer is:
mortal
'fatal' is the definition.
('mortal' can be similar in meaning to 'fatal')
'lesson about time' is the wordplay.
'lesson' becomes 'moral' ('moral' can be a synonym of 'lesson').
'about' means one lot of letters goes inside another.
'time' becomes 't'.
'moral' placed around 't' is 'MORTAL'.
(Other definitions for mortal that I've seen before include "Liable to cause death" , "Certain to die" , "Fatal" , "Causing death" , "Very drunk < person" .)