Feeling less sun, ultimately, by dark shade (6)
I believe the answer is:
number
'feeling less' is the definition.
(more numb)
'sun ultimately by dark shade' is the wordplay.
'ultimately' suggests the final letters.
'by dark shade' becomes 'umber' (I can't justify this - if you can you should believe this answer much more).
The final letter of 'sun' is 'n'.
'n'+'umber'='NUMBER'
Can you help me to learn more?
(Other definitions for number that I've seen before include "Count, musical item" , "27, say" , "Symbol indicating how many" , "Count; amount" , "Part of musical" .)