Cheer as Best is substituted for United (8)
I believe the answer is:
optimism
'cheer' is the definition.
(thesaurus)
'best is substituted for united' is the wordplay.
I cannot quite see how this works, but
'is' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for optimism that I've seen before include "Hopefulness" , "Confident outlook" , "Looking on the bright side" , "Hopeful nature" , "The tendency to always expect the best" .)