Cinema shortened Christmas party after reflection (5)
I believe the answer is:
odeon
'cinema' is the definition.
(I've seen this before)
'shortened christmas party after reflection' is the wordplay.
'shortened' means to remove the last letter.
'christmas' becomes 'Noel' (Noel can mean Christmas).
'party' becomes 'do' (do can informally mean a party).
'after reflection' says the letters should be written in reverse.
'noel' with its last letter taken off is 'noe'.
'noe'+'do'='noedo'
'noedo' written backwards gives 'ODEON'.
(Other definitions for odeon that I've seen before include "old theatre" , "Ancient Greek concert hall" , "Hal" , "Large UK cinema chain" , "dramas here" .)