High earners taking in southern mountain air quality (8)
I believe the answer is:
rareness
'mountain air quality' is the definition.
(mountain air is rare)
'high earners taking in southern' is the wordplay.
'high' is an anagram indicator (like drunk).
'taking in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'earners' with letters rearranged gives 'rarenes'.
'rarenes' placed around 's' is 'RARENESS'.
(Other definitions for rareness that I've seen before include "Scarcity" , "Uncommon quality" , "Unusual quality" , "State of being uncommon or very scarce" .)