Decisive moment for start and end of career is repeated (6)
I believe the answer is:
crisis
'decisive moment' is the definition.
(I've seen this before)
'start and end of career is repeated' is the wordplay.
'start and end of' says to hollow out the word (remove centre letters) (only the first and last letter remain).
'repeated' says to double the letters.
'career' with its centre taken out is 'cr'.
'is' doubled is 'isis'.
'cr'+'isis'='CRISIS'
'for' is the link.
(Other definitions for crisis that I've seen before include "Emergency - decision time" , "a difficult situation" , "Emergency (in midlife?)" , "Crucial stage in a series of events" , "Time of decision or acute danger" .)