Emperor, okay at heart, lives on in retirement (6)
I believe the answer is:
kaiser
'emperor' is the definition.
(German term for emperor)
'okay at heart lives on in retirement' is the wordplay.
'at heart' says to take the centre.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'on' becomes 're' ('on' can be mean 'with reference to').
'in retirement' is a reversal indicator (retire can mean to go back or backwards).
The central letters of 'okay' are 'ka'.
're' in reverse letter order is 'er'.
'ka'+'is'+'er'='KAISER'
(Other definitions for kaiser that I've seen before include "Emperor of Austria or Germany until 1918" , "Austrian bigwig" , "Former German emperor" , "Old German emperor like Bill" , "Ex-ruler of Germany" .)