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' indicates putting letters inside (inviting in, as for dinner).
'alien' becomes 'et' (extra terrestrial).
'et'+'in'='etin'
'meg' placed around 'etin' is 'MEETING'.
(Other definitions for meeting that I've seen before include "consultation" , "Rendezvous" , "Satisfying" , "Assembly" , "Conference" .)