Half a constellation appears pale blue (4)
I believe the answer is:
aqua
'pale blue' is the definition.
(aqua is a shade of blue)
'half a constellation' is the wordplay.
'half a' indicates taking half.
'constellation' becomes 'aquarius' (Aquarius is an example).
'aquarius' halved is 'AQUA'.
'appears' acts as a link.
(Other definitions for aqua that I've seen before include "shade" , "Latin for water" , "- - Libra" , "Light bluish-green -- water" , "-- fortis, 'strong water'" .)