Swiftly covering boil up somewhere above ground (9)
I believe the answer is:
aerospace
'somewhere above ground' is the definition.
(the atmosphere and the space beyond it)
'swiftly covering boil up' is the wordplay.
'swiftly' becomes 'apace' (synonyms).
'covering' is an insertion indicator.
'boil' becomes 'sore' (boil can mean a sore or inflammation).
'up' is a reversal indicator (in down clue: letters go upwards).
'sore' reversed gives 'eros'.
'apace' enclosing 'eros' is 'AEROSPACE'.
(Other definitions for aerospace that I've seen before include "Branch of technology including aviation" , "The aviation world" , "zone near Earth" , "Technology of planes, rockets, etc" , "Opera case (anag)" .)