Party subordinate in Canada once (8)
I believe the answer is:
dominion
'canada once' is the definition.
(I've seen this in another clue)
'party subordinate' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'subordinate' becomes 'minion' (I've seen this before).
'do'+'minion'='DOMINION'
'in' is the link.
(Other definitions for dominion that I've seen before include "Area of control" , "Authority to rule over and control people" , "Supremacy" , "power" , "Realm" .)