Sign put up in star billing (5)
I believe the answer is:
libra
'sign' is the definition.
(sign of the zodiac)
'up in star billing' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates the answer is hidden within the clue.
'arbil' is hidden within 'star billing'.
'arbil' in reverse letter order is 'LIBRA'.
'put' acts as a link.
(Other definitions for libra that I've seen before include "Zodiacal sign" , "Old pound" , "Constellation near Virgo" , "Seventh sign of the Zodiac" , "Balance constellation" .)