Affirm boast about unfinished government order (9)
I believe the answer is:
predicate
'affirm' is the definition.
(I've seen this before)
'boast about unfinished government order' is the wordplay.
'boast' becomes 'prate'.
'about' indicates putting letters inside.
'unfinished' means to remove the last letter.
'government order' becomes 'edict' (I've seen this in another clue).
'edict' with its final letter removed is 'edic'.
'prate' enclosing 'edic' is 'PREDICATE'.
(Other definitions for predicate that I've seen before include "subject will be given this for sentence" , "It says something about the subject in a sentence" , "Assert, affirm" , "Describes part of a sentence" , "Grammatically, the statement made about a subject" .)