Since start of Cup Final a ‘new' heartless City's domination (10)
I believe the answer is:
ascendancy
'domination' is the definition.
(I've seen this before)
'since start of cup final a new heartless city's' is the wordplay.
I cannot quite see how this works, but
'since' could be 'as' (both can mean 'because') and 'as' is found in the answer.
'final' could be 'end' (synonyms) and 'end' is located in the answer.
'a' could be 'an' and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ascendancy that I've seen before include "Influential position" , "Dominating influence (A, not E)" , "Candy canes (anag.)" , "upper hand" , "Domination" .)