Dark except when sierra is to the west (7)
I believe the answer is:
sunless
'dark' is the definition.
(I've seen this in another clue)
'except when sierra is to the west' is the wordplay.
I cannot really see how this works, but
'except' could be 'unless' (I've seen this in another clue) and 'unless' is located in the answer.
'sierra' could be 's' (phonetic alphabet: alpha, bravo, charlie etc.) and 's' is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sunless that I've seen before include "dark" , "Unlit by solar light" , "gloomy" , "Clouded over" , "Offering nothing for a tanner" .)