Group leaders to end talks after month (5)
'group' is the definition.
(group of eight musicians)
'leaders to end talks after month' is the wordplay.
'leaders to' suggests taking the first letters (I've seen 'leader to' mean this (the letter which leads the word)).
'after' means one lot of letters go next to another.
'month' becomes 'oct' (abbreviation for October).
The first letters of 'end talks' is 'et'.
'et' put after 'oct' is 'OCTET'.
(Other definitions for octet that I've seen before include "Set of eight", "Group of eight players", "Chamber ensemble, a Mendelssohn favourite", "music group", "Mendelssohn wrote this".)