Met base, heartless, insane gangster (6)
I believe the answer is:
yardie
'gangster' is the definition.
(yardie is a kind of gangster)
'met base heartless insane' is the wordplay.
'met' is a charade indicator (letters next to each other).
'base' becomes 'yard' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'heartless' means to remove the middle letters.
'insane' with its middle removed is 'ie'.
'yard' put next to 'ie' is 'YARDIE'.
Can you help me to learn more?
(Other definitions for yardie that I've seen before include "Gangster (originally Jamaican)" , "West Indian gangster" , "West Indian gang member" .)