Plant beginning to die - is mistake admitted? (6)
I believe the answer is:
derris
'plant' is the definition.
(derris is a kind of plant)
'beginning to die is mistake admitted?' is the wordplay.
'beginning to' indicates taking the first letters.
'mistake' becomes 'err' ('err' can be a synonym of 'mistake'**).
'admitted?' is an insertion indicator.
The first letter of 'die' is 'd'.
'd'+'is'='dis'
'dis' placed around 'err' is 'DERRIS'.
(Other definitions for derris that I've seen before include "lethal powder" , "Woody plant" .)