Short month with alien group (5)
I believe the answer is:
octet
'group' is the definition.
(octet is a kind of group)
'short month with alien' is the wordplay.
'short month' becomes 'oct' (Oct is an example. I am not sure about the 'short' bit.**).
'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 "A number of musicians" , "Group that plays" , "Group of eight, say singers" , "Musical composition for eight performers" , "Set of eight" .)