It's true about Capone (4)
I believe the answer is:
real
'it's true' is the definition.
(similar in meaning)
'about capone' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'capone' becomes 'Al' (gangster Al Capone).
're'+'al'='REAL'
(Other definitions for real that I've seen before include "very American" , "Physically existing" , "Unaffected" , "Spanish football team" , "Madrid FC; genuine" .)