Butterflies started nibbling every random violet, except stem (6)
I believe the answer is:
nerves
'butterflies' is the definition.
(I've seen this in another clue)
'started nibbling every random violet except stem' is the wordplay.
'started' suggests taking the first letters.
The initial letters of 'nibbling every random violet except stem' is 'NERVES'.
(Other definitions for nerves that I've seen before include "Bodily fibres conveying sensations" , "Fibres that convey messages to the brain" , "Feeling of anxiety" , "causing some anxiety" , "They carry information between brain and body" .)