Happier when Starmer after change of heart supports leaders of leftist unitary councils (7)
I believe the answer is:
luckier
'happier' is the definition.
(happy can mean lucky or fortunate)
'heart supports leaders of leftist unitary councils' is the wordplay.
'heart' becomes 'kier' (I am not sure about this - if you are sure you should believe this answer much more).
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'leaders of' indicates taking the first letters.
The first letters of 'leftist unitary councils' is 'luc'.
'kier' put after 'luc' is 'LUCKIER'.
'when starmer after change of' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for luckier that I've seen before include "More fortunate" , "More jammy" .)