Run to stop a boy in crime (5)
I believe the answer is:
arson
'crime' is the definition.
(I've seen this before)
'run to stop a boy' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'to stop' indicates putting letters inside.
'boy' becomes 'son' (synonyms).
'a'+'son'='ason'
'r' going into 'ason' is 'ARSON'.
'in' acts as a link.
(Other definitions for arson that I've seen before include "Criminal pyromania" , "Crime of fire-setting (5)" , "Incendiary act" , "Fire-raising crime" , "Torching" .)