Being tense during lesson (6)
I believe the answer is:
mortal
'being' is the definition.
(mortal is a kind of being)
'tense during lesson' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'during' means one lot of letters goes inside another.
'lesson' becomes 'moral' ('moral' can be a synonym of 'lesson').
't' going into 'moral' is 'MORTAL'.
(Other definitions for mortal that I've seen before include "Sound a wound is deadly" , "Fatal" , "Certain to die" , "Very drunk < person" , "liable to die" .)