Obscure setter is capped by duke (3)
I believe the answer is:
dim
'obscure' is the definition.
(both can mean dark)
'setter is capped by duke' is the wordplay.
'setter' becomes 'I' (the person who set this crossword).
'is capped' becomes 'm' (I can't justify this - if you can you should believe this answer much more).
'by' is a charade indicator (letters next to each other).
'duke' becomes 'd' (abbreviation).
'i'+'m'='im'
'im' after 'd' is 'DIM'.
Can you help me to learn more?
(Other definitions for dim that I've seen before include "Badly lit or not so bright" , "out of mind?" , "Not brilliant" , "reduce volume" , "Not very bright or intelligent" .)