Short poem introducing his approach is killer understatement! (5)
I believe the answer is:
haiku
'short poem' is the definition.
(I've seen this in another clue)
'introducing his approach is killer understatement' is the wordplay.
'introducing' indicates taking the first letters.
The initial letters of 'his approach is killer understatement' is 'HAIKU'.
(Other definitions for haiku that I've seen before include "Epigrammatic verse" , "Jap. 17-syllable verse" , "Seventeen-syllable Japanese poem" , "17-syllable Japanese verse form" , "Japanese three-line poem of 17 syllables" .)