A Republican lad's crime (5)
I believe the answer is:
arson
'crime' is the definition.
(I've seen this before)
'a republican lad's' is the wordplay.
'republican' becomes 'r' (abbreviation).
'lad' becomes 'son'.
'a'+'r'+'son'='ARSON'
(Other definitions for arson that I've seen before include "criminal fire-raising" , "Malicious fire-raising" , "Crime of fire raising" , "Roans (anag)" , "Act of a pyromaniac" .)