Group ends talks that have only started after a month (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
'ends talks that have only started after a month' is the wordplay.
'that have only started' says to take the initial letters.
'after' means one lot of letters go next to another.
'a month' becomes 'oct' (abbreviation for October).
The initial letters of 'ends talks' is 'et'.
'et' put after 'oct' is 'OCTET'.
(Other definitions for octet that I've seen before include "Lines of verse" , "Eight people together" , "Musical work for eight" , "Medieval Latin hymn" , "Eightsome" .)