Crime of murder finally admitted by a boy (5)
I believe the answer is:
arson
'crime' is the definition.
(I've seen this before)
'murder finally admitted by a boy' is the wordplay.
'finally' indicates one should take the final letters.
'admitted by' means one lot of letters goes inside another.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
The last letter of 'murder' is 'r'.
'a'+'son'='ason'
'r' placed within 'ason' is 'ARSON'.
'of' is the link.
(Other definitions for arson that I've seen before include "criminal fire-raising" , "Fire-raiser's crime" , "major crime" , "matchplay, say?" , "Torching" .)