Ziggy Stardust possibly short-changed over advance (5,3)
I believe the answer is:
alter ego
'ziggy stardust' is the definition.
(I know that ziggy stardust is a alter ego)
'short-changed over advance' is the wordplay.
'short' means to remove the last letter.
'changed' becomes 'altered' ('alter' can be a synonym of 'change').
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'advance' becomes 'go' (to advance is to go forward).
'altered' with its last letter removed is 'altere'.
'altere'+'go'='ALTER EGO'
'possibly' acts as a link.
(Other definitions for alter ego that I've seen before include "Is it Kent" , "Hidden self or very close friend" , "Other self - large toe (anag)" , "Go, relate to other self" , "A second personality or intimate friend" .)