More understandable when there’s better visibility (7)
I believe the answer is:
clearer
'more understandable' is the definition.
('clear' can be similar in meaning to 'understandable')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for clearer that I've seen before include "Less clouded, more well-defined" , "Less obscured" , "More distinct" , "Less overcast" , "More intelligible, less cloudy" .)