A boy harbours criminal since uprising (9)
I believe the answer is:
ascension
'uprising' is the definition.
(thesaurus)
'a boy harbours criminal since' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'harbours' means one lot of letters goes inside another.
'criminal' indicates anagramming the letters.
'since' anagrammed gives 'censi'.
'son' enclosing 'censi' is 'scension'.
'a'+'scension'='ASCENSION'
(Other definitions for ascension that I've seen before include "distance in hours measured by this, right?" , "Canonise on the way up to heaven" , "Canonises on Christ's way to heaven" , "religious feast" , "holy day" .)