Following, not starting to be troubling (6)
I believe the answer is:
ailing
'troubling' is the definition.
('ail' can be a synonym of 'trouble')
'following not starting' is the wordplay.
'following' becomes 'tailing' (similar in meaning).
'not starting' means to remove the first letter.
'tailing' with its first letter taken away is 'AILING'.
'to be' acts as a link.
(Other definitions for ailing that I've seen before include "Unwell, in poor condition" , "In poor health" , "Infertile" , "Sickly, poorly" , "Suffering" .)