Announce fantastic reduction (9)
I believe the answer is:
introduce
'announce' is the definition.
(thesaurus)
'fantastic reduction' is the wordplay.
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'reduction' is an anagram of 'INTRODUCE'.
(Other definitions for introduce that I've seen before include "To add" , "Launch" , "Present to another or bring up for discussion" , "Bring in - cause to meet" , "Bring to notice or make known" .)