He's a criminal protecting the heart of Cardiff underworld (5)
I believe the answer is:
hades
'underworld' is the definition.
('Hades' can be a synonym of 'underworld')
'he's a criminal protecting the heart of cardiff' is the wordplay.
'criminal' is an anagram indicator.
'protecting' is an insertion indicator.
'the heart of' means to look at the middle letters.
The middle letter of 'cardiff' is 'd'.
'hes'+'a'='hesa'
'hesa' anagrammed gives 'haes'.
'haes' placed around 'd' is 'HADES'.
(Other definitions for hades that I've seen before include "Greek god of the underworld" , "Classic hell in the shade" , "gloomy place" , "The underworld where the souls of the dead live" , "The nether world, hell" .)