Mistress admitted having bottom scratched by king (5)
I believe the answer is:
owner
'mistress' is the definition.
(thesaurus)
'admitted having bottom scratched by king' is the wordplay.
'admitted' becomes 'owned' (I've seen this before).
'having bottom scratched' means to remove the last letter.
'by' means one lot of letters go next to another.
'king' becomes 'r' (r is abbreviation for rex).
'owned' with its final letter taken off is 'owne'.
'owne'+'r'='OWNER'
(Other definitions for owner that I've seen before include "One that secured" , "Legal possessor" , "Holder, proprietor" , "The one in lawful possession" , "landlord" .)