Extraordinarily, underdog is well-balanced and sensible (8)
I believe the answer is:
grounded
'well-balanced and sensible' is the definition.
I can't tell whether this defines the answer.
'extraordinarily underdog' is the wordplay.
'extraordinarily' indicates anagramming the letters (I've seen this in other clues).
'underdog' with letters rearranged gives 'GROUNDED'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for grounded that I've seen before include "Based; forbidden to fly" , "Punished teenager by confining to house - poor underdog" , "Not allowed to fly" , "Earthed" , "Prevented from flying" .)