Hear King's changing, making reduction (9)
I believe the answer is:
shrinkage
'reduction' is the definition.
(thesaurus)
'hear king's changing' is the wordplay.
'changing' indicates anagramming the letters.
'hear'+'kings'='hearkings'
'hearkings' with letters rearranged gives 'SHRINKAGE'.
'making' acts as a link.
(Other definitions for shrinkage that I've seen before include "Process by which something becomes smaller" , "waste allowance" , "Asking her to get loss by wastage" , "Size reduction" , "Reduction in volume" .)