Stop protecting Capone – it's very common (9)
I believe the answer is:
prevalent
'very common' is the definition.
'prevalent' can be an answer for 'common' (prevalent can mean common or widespread). I am unsure of the 'very' bit.
'stop protecting capone' is the wordplay.
'stop' becomes 'prevent' (stop can mean to prevent something).
'protecting' is an insertion indicator (inserted letters are held or protected).
'capone' becomes 'Al' (gangster Al Capone).
'prevent' going around 'al' is 'PREVALENT'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for prevalent that I've seen before include "Common" , "Ruling" , "Commonplace" , "Widely accepted" , "powerful" .)