Decoration or star, new on top of tree (8)
I believe the answer is:
ornament
'decoration' is the definition.
(I know that decoration can be written as ornament)
'or star new on top of tree' is the wordplay.
'star' becomes 'name' (I've seen this in other clues).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'on top of' suggests taking the first letters.
The initial letter of 'tree' is 't'.
'or'+'name'+'n'+'t'='ORNAMENT'
(Other definitions for ornament that I've seen before include "Embellish" , "It's used to adorn" , "trill?" , "Something decorative" , "deck" .)