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' says to put letters next to each other.
'alien' becomes 'et' (extraterrestrial).
'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" .)