On welcoming first of invitees, call up vassal (8)
I believe the answer is:
liegeman
'vassal' is the definition.
(I know that vassal can be written as liegeman)
'on welcoming first of invitees call up' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'first of' says to take the initial letters.
'call' becomes 'name' (as in calling or naming someone something).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The first letter of 'invitees' is 'i'.
'name' in reverse letter order is 'eman'.
'leg' placed around 'i' is 'lieg'.
'lieg'+'eman'='LIEGEMAN'
(Other definitions for liegeman that I've seen before include "A loyal person" , "Vassal, subject" , "Person serving lord" , "Loyal follower" .)