Former old king’s minister (6)
I believe the answer is:
pastor
'minister' is the definition.
('pastor' can be a synonym of 'minister')
'former old king's' is the wordplay.
'former' becomes 'past' ('past' can be similar in meaning to 'former').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'king' becomes 'r' (r is abbreviation for rex).
'past'+'o'+'r'='PASTOR'
(Other definitions for pastor that I've seen before include "Minister; shepherd" , "To spar with clergyman" , "who may provide a service" , "Padre perhaps" , "Churchman" .)