All set for reform at end of December? Outstanding! (7)
I believe the answer is:
stellar
'outstanding' is the definition.
(I've seen this in another clue)
'all set for reform at end of december?' is the wordplay.
'for reform' indicates an anagram (I've seen this in other clues).
'at end of' says to take the final letters.
The last letter of 'december' is 'r'.
'all'+'set'='allset'
'allset' with letters rearranged gives 'stella'.
'stella'+'r'='STELLAR'
(Other definitions for stellar that I've seen before include "Excellent - shining bright?" , "Top-notch" , "Astral" , "Wonderful" , "High-class" .)