Month with alien group (5)
I believe the answer is:
octet
'group' is the definition.
(group of eight musicians)
'month with alien' is the wordplay.
'month' becomes 'oct' (abbreviation for October).
'with' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extra terrestrial).
'oct'+'et'='OCTET'
(Other definitions for octet that I've seen before include "Medieval Latin hymn" , "Music for eight" , "What takes members up to eighth" , "Group of eight musicians" , "Musical work for eight" .)