Group leaders to end talks after month (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
'leaders to end talks after month' is the wordplay.
'leaders to' indicates taking the first letters (I've seen 'leader to' mean this (the letter which leads the word)).
'after' says to put letters next to each other.
'month' becomes 'oct' (abbreviation for October).
The initial letters of 'end talks' is 'et'.
'et' after 'oct' is 'OCTET'.
(Other definitions for octet that I've seen before include "Musical work for eight" , "Eightsome" , "Work for eight players" , "Mendelssohn or Schubert piece" , "Played" .)