Quasar exploding round one high-class constellation (8)
I believe the answer is:
aquarius
'constellation' is the definition.
(Aquarius is an example)
'quasar exploding round one high-class' is the wordplay.
'exploding' is an anagram indicator.
'round' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'high class' becomes 'u'.
'quasar' anagrammed gives 'aquars'.
'i'+'u'='iu'
'aquars' placed around 'iu' is 'AQUARIUS'.
(Other definitions for aquarius that I've seen before include "Sign of the water-carriers" , "Sign of the Water-carrier - one of 12 Across" , "Eleventh sign of the Zodiac" , "A 23 ac" , "The Water Bearer" .)