I cut arm at training, which is distressing (9)
I believe the answer is:
traumatic
'is distressing' is the definition.
(I've seen this before)
'i cut arm at training' is the wordplay.
'training' indicates an anagram (to train can mean to grow something into a shape).
'i'+'cut'+'arm'+'at'='icutarmat'
'icutarmat' anagrammed gives 'TRAUMATIC'.
'which' acts as a link.
(Other definitions for traumatic that I've seen before include "Psychologically painful" , "Deeply disturbing" , "Direly shocking" , "Deeply distressing" , "Emotionally shocking" .)