Extraordinarily posh retreats somewhere far away (12)
I believe the answer is:
stratosphere
'somewhere far away' is the definition.
(the stratosphere is high up in the sky)
'extraordinarily posh retreats' is the wordplay.
'extraordinarily' is an anagram indicator (I've seen this in other clues).
'posh'+'retreats'='poshretreats'
'poshretreats' is an anagram of 'STRATOSPHERE'.
(Other definitions for stratosphere that I've seen before include "Part of the air" , "High level above earth" , "Layer of high air around the earth" , "Atmospheric layer" , "Layer above the earth where temperature generally increases with height" .)