Japanese verse has as its key unity, primarily (5)
I believe the answer is:
haiku
'japanese verse' is the definition.
(I've seen this before)
'has as its key unity primarily' is the wordplay.
'primarily' indicates taking the first letters.
The initial letters of 'has as its key unity' is 'HAIKU'.
(Other definitions for haiku that I've seen before include "Japanese three-line poem of 17 syllables" , "Jap 17 syllable poem" , "Seventeen-syllable Japanese poem" , "Stylised Japanese poem of 17 syllables" , "17-syllable Japanese verse form" .)