Concluded there's a need to change before the first of December (5)
I believe the answer is:
ended
'concluded' is the definition.
(synonyms)
'a need to change before the first of december' is the wordplay.
'to change' is an anagram indicator.
'before' means one lot of letters go next to another.
'the first of december' becomes 'd' (1st letter of 'december').
'need' with letters rearranged gives 'ende'.
'ende'+'d'='ENDED'
'there's' acts as a link.
(Other definitions for ended that I've seen before include "Concluded" , "wound up" , "Over; complete" , "Ceased" , "Stopped" .)