Party underling in ascendancy (8)
I believe the answer is:
dominion
'ascendancy' is the definition.
(I know that dominion is a type of ascendancy)
'party underling' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'underling' 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" , "Government" , "territory" , "Territorial possession controlled by a ruling state" , "Supremacy" .)