It's cloudless, that's obvious (5)
I believe the answer is:
clear
I believe this is a double definition.
'it's cloudless' is the first definition.
(I've seen this in another clue)
'obvious' is the second definition.
(similar in meaning)
'that's' is the link.
(Other definitions for clear that I've seen before include "negotiate" , "Transparent, easy to see through" , "Fair - innocent" , "bound over" , "Understandable" .)