Conclusion: don't start improving (6)
I believe the answer is:
ending
'conclusion' is the definition.
(I know that conclusion can be written as ending)
'don't start improving' is the wordplay.
'don't start' means to remove the first letter.
'improving' becomes 'mending' (mending is a kind of improving).
'mending' with its initial letter taken away is 'ENDING'.
(Other definitions for ending that I've seen before include "Last part" , "Stopping" , "Finish" , "Finale" , "Closing down" .)