Month with alien ensemble (5)

I believe the answer is:
octet
'ensemble' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'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'
Can you help me to learn more?
(Other definitions for octet that I've seen before include "Musical composition for eight performers" , "Work for eight players" , "Mendelssohn wrote this" , "Mendelssohn or Schubert piece" , "A group of eight instruments or voices" .)
