Completion of old French coin into chaos (9)
I believe the answer is:
execution
'completion' is the definition.
(thesaurus)
'old french coin into chaos' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'french coin' becomes 'ecu' (I've seen this in another clue).
'chaos' indicates anagramming the letters.
'into' is an anagram of 'tion'.
'ex'+'ecu'+'tion'='EXECUTION'
'of' acts as a link.
(Other definitions for execution that I've seen before include "Queen of Hearts' order" , "Topping" , "Eg, beheading" , "style of musical performance" , "The carrying out of" .)