Margaret welcomes alien in tryst (7)
I believe the answer is:
meeting
'tryst' is the definition.
(thesaurus)
'margaret welcomes alien in' is the wordplay.
'margaret' becomes 'meg'.
'welcomes' is an insertion indicator (in sense of 'invites in').
'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" , "Conference" , "Encountering" , "fixture" , "Rendevous" .)