Ninth month with sci-fi film group (6)
I believe the answer is:
septet
'group' is the definition.
(septet is a kind of group)
'ninth month with sci-fi film' is the wordplay.
'ninth month' becomes 'sept'.
'with' is a charade indicator (letters next to each other).
'sci-fi film' becomes 'et' (I've seen this in other clues).
'sept'+'et'='SEPTET'
(Other definitions for septet that I've seen before include "A group of seven, say singers" , "A set of seven" , "Group of seven players" , "G7" , "Odd number of players performing" .)