Reportedly excellent masterstroke to produce short verse form (5)
I believe the answer is:
'verse form' is the definition.
(I know that haiku is a type of verse form)
'reportedly excellent masterstroke to produce' is the wordplay.
I cannot quite see how this works, but
'excellent' could be 'ai' (resembles 'A1', old-fashioned term for excellent) and 'ai' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'short' is the link.
Can you help me to learn more?
(Other definitions for haiku that I've seen before include "Stylised Japanese poem of 17 syllables" , "Jap. 17-syllable verse" , "Japanese 3-line poem of 17 syllables" , "ode" , "Short Japanese poem" .)