Payment including king's ransom (4)
I believe the answer is:
free
'ransom' is the definition.
(thesaurus)
'payment including king's' is the wordplay.
'payment' becomes 'fee' (I've seen this before**).
'including' is an insertion indicator.
'king' becomes 'r' (r is abbreviation for rex).
'fee' enclosing 'r' is 'FREE'.
(Other definitions for free that I've seen before include "unoccupied)" , "Exempt" , "Untrammelled" , "Costing nothing" , "Gratis, for nothing" .)