See where sun sets closest to horizon (6)
I believe the answer is:
lowest
'sun sets closest to horizon' is the definition.
I can't judge whether this defines the answer.
'see where' is the wordplay.
'see' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'where' becomes 'west' (I can't justify this - if you can you should give a lot more credence to this answer).
'lo'+'west'='LOWEST'
Can you help me to learn more?
(Other definitions for lowest that I've seen before include "Nearest the ground" , "very close to the ground" , "Owlets at the bottom level" , "Furthest down" , "Given maximum reduction" .)