Go in to welcome Conservative hosting union meeting (9)
I believe the answer is:
'meeting' is the definition.
('encounter' can be a synonym of 'meeting')
'go in to welcome conservative hosting union' is the wordplay.
'go in to' becomes 'enter' (I've seen this before).
'welcome' means one lot of letters goes inside another (in sense of 'inviting in').
'conservative' becomes 'con' (I've seen this in other clues).
'hosting' is an insertion indicator.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'con' enclosing 'u' is 'coun'.
'enter' going around 'coun' is 'ENCOUNTER'.
(Other definitions for encounter that I've seen before include "Struggle" , "fight" , "battle" , "Unexpected meeting" , "Meet by chance with encore nut" .)