He succeeds, having a heart of stone (7)
I believe the answer is:
winston
'he' is the definition.
'succeeds having a heart of stone' is the wordplay.
'succeeds' becomes 'wins'.
'having' means one lot of letters go next to another.
'a heart of' says to take the centre.
The middle of 'stone' is 'ton'.
'wins'+'ton'='WINSTON'
(Other definitions for winston that I've seen before include "Prime minister" , "Man" , "Statesman" , "-- Churchill, UK PM" , "11 [LAWMAKER]" .)