Undergoes combustion (5)
I believe the answer is:
burns
'undergoes combustion' is the definition.
('burn' can be a synonym of 'undergo combustion')
This is the entire clue.
(Other definitions for burns that I've seen before include "Is on fire" , "experiences strong emotion" , "Cremates -- Scottish poet, d.1796" , "Scottish lyricist" , "Rabbie - - (poet)" .)