Six against one, humbled ultimately by head's clarity (9)
I believe the answer is:
vividness
'six against one humbled ultimately' is the definition.
I can't tell whether this definition defines the answer.
'by head's clarity' is the wordplay.
'by' is a charade indicator (letters next to each other).
'head' becomes 'ness' (type of geographical point).
'clarity' becomes 'vivid' (I can't explain this - if you can you should believe this answer much more).
'ness' after 'vivid' is 'VIVIDNESS'.
Can you help me to learn more?
(Other definitions for vividness that I've seen before include "Intense depth (of colour)" , "Intense brightness" .)