Source of troubles past still happening (5)
I believe the answer is:
event
'happening' is the definition.
(I know that happening can be written as event)
'source of troubles past still' is the wordplay.
'source of' indicates taking the first letters.
'past' says to put letters next to each other.
'still' becomes 'even' (synonyms).
The initial letter of 'troubles' is 't'.
't' put after 'even' is 'EVENT'.
(Other definitions for event that I've seen before include "Occurence, happening" , "Something happening" , "Episode" , "Incident - final outcome" , "riding competition" .)