King who upset his aunt in church (6)
I believe the answer is:
canute
'king' is the definition.
(King Canute, 11th-century king)
'upset his aunt in church' is the wordplay.
'upset' shows that the letters should be reversed in order.
'his aunt' becomes 'tuna' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'church' becomes 'CE' (abbreviation for Church of England).
'tuna' reversed gives 'anut'.
'anut' going inside 'ce' is 'CANUTE'.
'who' is the link.
Can you help me to learn more?
(Other definitions for canute that I've seen before include ""Back," he said" , "Sea-defying king of England" , "King who ordered the tide to retreat" , "Old king of England who ordered the tide to go out" , "King defeated by tide" .)