Loose in joyous part of universe (6)
I believe the answer is:
galaxy
'part of universe' is the definition.
'loose in joyous' is the wordplay.
'loose' becomes 'lax' ('lax' is associated in meaning with 'loose').
'in' means one lot of letters goes inside another.
'joyous' becomes 'gay' ('gay' can be similar in meaning to 'joyous').
'lax' going within 'gay' is 'GALAXY'.
(Other definitions for galaxy that I've seen before include "Lots of stars" , "many stars" , "Huge collection of stars" , "Large star system" , "Stellar system" .)