Margaret entertaining alien in assembly (7)
I believe the answer is:
meeting
'assembly' is the definition.
(I know that meeting is a type of assembly)
'margaret entertaining alien in' is the wordplay.
'margaret' becomes 'meg' (short for Margaret).
'entertaining' means one lot of letters goes inside another.
'alien' becomes 'et' (extra terrestrial).
'et'+'in'='etin'
'meg' enclosing 'etin' is 'MEETING'.
(Other definitions for meeting that I've seen before include "consultation" , "Rendezvous" , "Satisfying" , "Assembly" , "Conference" .)