Human molar cracked with temperature (6)
I believe the answer is:
mortal
'human' is the definition.
('mortal' can be a synonym of 'human')
'molar cracked with temperature' is the wordplay.
'cracked with' indicates anagramming the letters.
'temperature' becomes 't' (abbreviation).
'molar' mixed up with 't' makes 'MORTAL'.
(Other definitions for mortal that I've seen before include "Destined to die" , "Human being, subject to death" , "Causing death" , "Subject to death, like all mankind" , "Human - transitory" .)