Couple of unknowns introduced by party host (6)
I believe the answer is:
galaxy
'host' is the definition.
(I've seen this before)
'couple of unknowns introduced by party' is the wordplay.
'couple of unknowns' becomes 'xy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'introduced by' says to put letters next to each other.
'party' becomes 'gala' (I've seen this in another clue).
'xy' put after 'gala' is 'GALAXY'.
Can you help me to learn more?
(Other definitions for galaxy that I've seen before include "Assemblage of stars" , "Spiral, elliptical, group of stars" , "Our solar system is contained within it" , "The Milky Way, for example" , "Splendid assemblage" .)