Old king is engaging with an insider (6)
I believe the answer is:
canute
'old king' is the definition.
(medieval Danish king)
'with an insider' is the wordplay.
'with' indicates putting letters inside.
'insider' becomes 'cute' (I can't justify this - if you can you should believe this answer much more).
'an' placed within 'cute' is 'CANUTE'.
'is engaging' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for canute that I've seen before include "The king who couldn't stop the tide" , "King who ordered the waves to go back" , "King of England (1016)" , "Old king of England who tried to stop the tide" , "English king who ordered waves to retreat" .)