Group meet regularly after month (5)
I believe the answer is:
octet
'group' is the definition.
(octet is a kind of group)
'meet regularly after month' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'after' is a charade indicator (letters next to each other).
'month' becomes 'oct' (Oct is an example**).
The alternate letters of 'meet' are 'et'.
'et' put after 'oct' is 'OCTET'.
(Other definitions for octet that I've seen before include "What takes members up to eighth" , "Group of eight musicians" , "Set of eight" , "Musical group" , "Chamber ensemble, a Mendelssohn favourite" .)