A boy harbours criminal since uprising (9)
I believe the answer is:
Here is my best explanation:
'uprising' is the definition.
'a boy harbours criminal since' is the subsidiary indication.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'harbours' indicates putting letters inside.
'criminal' indicates anagramming the letters.
'since' with letters rearranged gives 'censi'.
'son' enclosing 'censi' is 'scension'.
(Other definitions for ascension that I've seen before include "Island in the South Atlantic", "A movement upward", "Canonises on Christ's way to heaven", "A way up" and "Canonise on the way up to heaven".)