Low northern celestial body (4)
I believe the answer is:
moon
'celestial body' is the definition.
(moon is a kind of celestial body)
'low northern' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'moo'+'n'='MOON'
(Other definitions for moon that I've seen before include "Orbiting body" , "Display one's buttocks publicly (slang)" , "Daydream - Europa of Jupiter, say" , "crescent perhaps" , "Appear listless" .)