Killer exercise for fast runner in Japan (6,5)
I believe the answer is:
bullet train
'fast runner in japan' is the definition.
(type of fast train in Japan)
'killer exercise' is the wordplay.
'killer' becomes 'bullet' (a bullet might kill someone).
'exercise' becomes 'train' (training is a kind of exercising).
'bullet'+'train'='BULLET TRAIN'
'for' acts as a link.
(Other definitions for bullet train that I've seen before include "it travels quickly" , "Get to work on it fast!" , "Japanese high-speed transport" .)