Dominion is true to form, ultimately (5)
I believe the answer is:
realm
'dominion' is the definition.
(I've seen this before)
'true to form ultimately' is the wordplay.
'true' becomes 'real' (similar in meaning).
'to' is a charade indicator (letters next to each other).
'ultimately' says to take the final letters.
The last letter of 'form' is 'm'.
'real'+'m'='REALM'
'is' is the link.
(Other definitions for realm that I've seen before include "where ruler holds sway?" , "Lamer (anag.)" , "monarch's responsibility" , "department" , "Land under a king or queen" .)