Most natural in Spain -- snooze showing bottom on top! (7)
I believe the answer is:
easiest
'most natural' is the definition.
('easy' can be similar in meaning to 'natural')
'spain snooze showing bottom on top' is the wordplay.
I cannot quite understand how this works, but
'spain' could be 'es' (es is an abbreviation for Spain) and 'es' is found in the answer.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for easiest that I've seen before include "Least difficult" , "most relaxed" , "Least hard" , "Requiring the least amount of effort" , "Most simple" .)