The old king was a devotee in church (6)
I believe the answer is:
canute
'the old king' is the definition.
(medieval Danish king)
'a devotee in church' is the wordplay.
'devotee' becomes 'nut' (both can mean an enthusiast).
'in' is an insertion indicator.
'church' becomes 'CE' (abbreviation for Church of England).
'a'+'nut'='anut'
'anut' inserted into 'ce' is 'CANUTE'.
'was' is the link.
(Other definitions for canute that I've seen before include "King who ordered the waves to go back" , "Ancient King who commanded the tide to retreat" , "Danish King of England, 1016-35" , "English king who commanded tide to go back" , "King of England (1016)" .)