Star of stage — in rep originally — on the up (5)
I believe the answer is:
rigel
'star' is the definition.
(Rigel is an example)
'stage in rep originally on the up' is the wordplay.
'stage' becomes 'leg' ('leg' can be a synonym of 'stage'**).
'originally' suggests taking the first letters.
'on' says to put letters next to each other.
'the up' is a reversal indicator (in down clue: letters go upwards).
The initial letters of 'in rep' is 'ir'.
'leg'+'ir'='legir'
'legir' reversed gives 'RIGEL'.
'of' acts as a link.
(Other definitions for rigel that I've seen before include "shiner" , "Brightest star in Orion" , "Orion's brightest star" .)